Hideo Ohashi, Prof. Emeritus

University of Tokyo & Kogakuin University
The Window to Society after Retirement from Engineering Education

　This top page is to transmit the proof that I am still alive and enjoying retired life, mainly to my relatives, friends and former students. A story or an event is selected from daily life and my thoughts and experience are introduced.

　Half a century has passed since I began living in Seijo, a town in Setagaya ward of Tokyo. Recently, I came across a table showing the population by age group in a newsletter published by Seijo Residents' Association. The data on the table sparked my interest in the town's demographic makeup. Seijo is a typical suburban residential area, renowned in Tokyo alongside Den-enchōfu. It spans from block 1 to block 9. The central area, home to many long-standing large estates, may differ significantly in resident profile from the peripheral area, where apartment complexes and condominiums are scattered.
　The Setagaya Ward Office publishes population data broken down by gender and age for each block of towns in the ward. Using the data as of January 2026, I set out to explore the characteristics of Seijo's population composition.
　Seijo’s total population is approximately 23,000, comprising 46% men and 54% women. A bar chart showing the age distribution is displayed above. Although I tried to glean specific insights about Seijo from this figure, I found few distinctive features. The trend of women living longer becomes pronounced after age 80, and the number of children aged 15 and under continues to decline, raising concerns about the future. These are phenomena likely occurring everywhere. However, the population valley seen in the age group around 30 stands out; this indicates that the outflow of this group far exceeds the inflow. This may well be the area's most significant demographic characteristic.
　Delving deeper, I investigated whether population composition varies by specific block within Seijo. Block 5 retains the most of the town's old-world atmosphere, with many large estates still standing. In contrast, block 8, where I live, is located about a 15-minute walk north of the station, and lies outside the zone subject to strict building height restrictions. Many high-rise apartments and condominiums in this block raise population density significantly. The difference in the density is reflected to the population: 1,999 residents in block 5 versus 3,777 in block 8. To facilitate a comparison, I converted the figures to a per-1,000-resident basis and plotted the demographic composition of two blocks on the line graph shown below.

　In demographic statistics, smaller populations exhibit greater year-to-year fluctuations, making it difficult to discern significant differences amidst such variability. Trying to identify underlying trends in fluctuations, it appears that the aging of the population is slightly more advanced in block 8, while no difference is observed regarding the decline in the birth rate. The population valley around the age of 30 is clearly deeper in block 8; this suggests a higher population mobility within this age group in newly developed areas. Regarding the elderly population, it was surprising to find a larger number of seniors in the newly developed area compared to the area with an older streetscape.
　Beyond mere statistical interest, the data reveals a great deal. Among the 1,675 men living in my block 8, I discovered that there are eleven individuals older than myself at 95 with the eldest being 101. This represents merely 0.6% of the male population. It starkly illustrates the reality that living a long life and having little time left are one and the same.
　After finishing the data analysis so far, I realized that to better understand the unique characteristics of Seijo, I needed to compare it with other administrative units. For this comparison, I selected Japan itself and Koto Ward, a waterfront area that stands in stark contrast to the upland character of Seijo. The population composition of three units per 1,000 residents is illustrated in the line graph below.

　A look at the data highlights the distinctive characteristics of Koto Ward. There is a significant influx of migrants who start self-standing in their twenties, resulting in a rapid growth of population. In contrast, Seijo experiences a sharp outflow of the same age group, creating a population valley that is deeper than anywhere else. Presumably, the high cost of housing in Seijo discourages young migrants from settling there.
　Seijo shows a population peak among those around the age of sixty, an age corresponding to peak earning years. This substantial high-income age group drives the wealthiness of the town. While Koto Ward lags behind in terms of population aging, the trend for Seijo is slightly less than the national average. The decline in the birthrate is a serious issue across the nation.
　I embarked on this study with great enthusiasm. Though findings were less than expected, I learned quite a lot on demography. Irregular fluctuations that obscure analysis tend to smooth out as the population size of the target area increases. When the scope is limited to the domestic Japanese population, changes due to migration do not occur unless inhabitants cross national borders. Line graphs showing age-based population composition become smoother and allow to identify specific peak and valley caused by wars, baby booms, or Hinoe-uma (an ominous year of superstition). Underlying trends like the declining birthrate and aging population also become clearly visible. Demographic studies focusing on small groups like the one I undertook here are rare, likely because irregular fluctuations can undermine the persuasiveness of the findings.
　Local governments accurately track their populations and regularly publish the data. Why not take a look at the population histogram for your own town or village and discover its unique characteristics?

Note: Imagine an isolated island with no migration in or out, where every woman gives birth to one boy and one girl at a set age, and every person lives to a fixed age without succumbing to accidents or disease. In this scenario, the island's population graph would form a rectangle, perfectly flat across the top before dropping sharply at the end. The complex variations in actual population composition arise entirely from deviations from these hypothetical conditions regarding birth, death, and migration.



→ Past Top Pages

　 　"From Daily Life"
　　　　　　 　1: Tracing family roots
　　　　　　 　2: The second last car
　　　　　　 　3: August 15. my memorial day
　　　　　 　　4: Battlefield rations
　　　　　 　　5: Begginer at the gym
　　　　　 　　6: Fuji View Hotel
　　　　　　 　7: Lesson in Hiragana writing by brush
　　　　　 　　8: Moving from old to new smartphone
　　　　　 　　9: Living alone in my second year
　　　　　 　　10: Welcomed by cherry blossom
　　　　　 　　11: Eden of peach blossom
　　　　　 　　12: Tama River Walk
　　　　　 　　13: Chichibu Pilgrimage
　　　　　 　　14: Bonds with local community
　　　　　 　　15: About my homepage
　　　　　 　　16: Annual report of athletic gym
　　　　　 　　17: Pilgrimage to Mt. Tsukuba
　　　　　 　　18: Reunion of alumni and classmates
　　　　　 　　19: Braking by left foot
　　　　　 　　20: Battlefield rations in the sequel
　　　　　 　　21: Granddaughter has come!
　　　　　 　　22: Visit to Kobayashi Residence
　　　　　 　　23: Wake-up exercise
　　　　　 　　24: Digital photo album
　　　　　 　　25: Seven tools for brushing teeth
　　　　　 　　26: Ambulance
　　　　　 　　27: Quick report on drive monitoring
　　　　　 　　28: Hearing aid
　　　　　 　　29: Jindai Botanical Park
　　　　　 　　30: 2020, a historic year
　　　　　 　　31: Topics of eye
　　　　　 　　32: Final report on drive monitoring
　　　　　 　　33: Cognitive ability test
　　　　　 　　34: PCI to prevent heart infarction
　　　　　 　　35: Ashikaga Flower Park
　　　　　 　　36: 90 years old supported by medication
　　　　　 　　37: 3 years with Subaru
　　　　　 　　38: 45 years living in Seijo
　　　　　 　　39: Caspian Sea yogurt
　　　　　 　　40: Home Museum of Aunt Zschäbitz
　　　　　 　　41: Reunion of university classmates
　　　　　 　　42: Okutama spar tour
　　　　　 　　43: New Year's greeting card
　　　　　 　　44: Implant
　　　　　 　　45: Fuchu Forest Park
　　　　　 　　46: Grave closing
　　　　　 　　47: Walking group of local community
　　　　　 　　48: How-to books for the elderly
　　　　　 　　49: Home altar
　　　　　 　　50: Such a life
　　　　　 　　51: Knee of the elderly
　　　　　 　　52: Metropolitan Kinuta Park
　　　　　 　　53: Shirakaba Art Museum of Kiyoharu
　　　　　 　　54: Haiku; Metasequoia
　　　　　 　　55: Three years with COVID-19
　　　　　 　　56: Battlefield rations in the 6th year
　　　　　 　　57: Hearing impairment
　　　　　 　　58: Shinjuku Station
　　　　　 　　59: Haiku with photograph
　　　　　 　　60: Moving to 5G smartphone
　　　　　 　　61: History of private cars
　　　　　 　　62: Setagaya Art Museum
　　　　　 　　63: FreeCell
　　　　　 　　64: Turbo
　　　　　 　　65: Hinata Temple of Yakushi
　　　　　 　　66: Moving of home computer
　　　　　 　　67: Amakusa tour
　　　　　 　　68: Hand craft as a hobby
　　　　　 　　69: Kitami Community Square
　　　　　 　　70: Photo-Haiku 2
　　　　　 　　71: Support scam abusing Trojan Horse
　　　　　 　　72: Bedside gym
　　　　　 　　73: Metropolitan Nogawa Park
　　　　　 　　74: Family funeral
　　　　　 　　75: Sudoku (Number Place)
　　　　　 　　76: Photo-Haiku 3
　　　　　 　　77: Ohyama Afri Shrine
　 　　　　　　78: Kunisaki Peninsula tour
　 　　　　　　79: The old school system is far away
　 　　　　　　80: Cooking group of local community
　 　　　　　　81: Motor fan group of local community
　 　　　　　　82: Directories disappear!
　 　　　　　　83: Tunnel ventilation
　 　　　　　　84: Hearing aid renewal
　 　　　　　　85: Photo-Haiku 4
　 　　　　　　86: Digital records of life
　 　　　　　　87: 80 years after the ceasefire
　 　　　　　　88: Diagnosis of walking ability
　 　　　　　　89: Tracing back in time
　 　　　　　　90: Tour around castles and potteries
　 　　　　　　91: Asset management
　 　　　　　　92: Email - Part 1
　 　　　　　　93: Email - Part 2
　 　　　　　　94: Fukushima accident and hydrogen explosion
　 　　　　　　95: Photo-Haiku 5

　 　"Memorable Scenes"
　 　 　 　 　　Series 1: Parade with Doctor hat on the head
　 　 　 　 　　Series 2: Calligraphy in a frame
　　　　　　　 Series 3: A young man from the Orient
　　　　　　　 Series 4: Sumo-stamping
　　　　　　　 Series 5: Waiting room of Shanghai Station
　　　　　　　 Series 6: Let's Walk Festival
　　　　　　　 Series 7: Last long-distance drive
　 　 　 　 　　Series 8: My Car No. 3
　 　 　 　 　　Series 9: Ski camp
　 　 　 　 　　Series 10: Safari experience
　　　 　　　　Series 11: First encounter with dogwood
　　　 　　　　Series 12: Mt. Fuji

　 　"Treasure Box of Words"
　 　 　 　 　　Series 1: Virtue brings profit
　 　 　 　 　　Series 2: Wait for a ripe moment
　 　 　 　 　　Series 3: Catch the symptoms
　 　 　 　 　　Series 4: Words are selected metaphors
　 　 　 　 　　Series 5: Start with two humans
　 　 　 　 　　Series 6: Optimism is a will, pessimism a mood
　 　 　 　 　　Series 7: Atop a summit, and another becomes visible
　 　 　 　　 　Series 8: We see only what we want to see
　　　　　　　 Series 9: Chanllenge is the privilage of the young
　　　　　　　 Series 10: Today is the first day of my life

　 　 Engineer Series 1: Who's engineer, juveniles' answer
　 　 　 　 　　Series 2: Corus of "We raise engineers!"
　 　 　 　 　　Series 3: From Nobel Prize to Gold Medal
　 　 　 　 　　Series 4: Professionals in three categories
　 　 　 　 　　Series 5: Various tasks in engineering
　 　 　 　 　　Series 6: Classification of EPs by IEA
　 　 　 　 　　Series 7: Science, technology and related professionals
　 　 　 　 　　Series 8: Proposal for the classification of engineers
　 　 　 　 　　Series 9: Revised proposal for the classification of engineers
　 　 　 　 　　Series 1 - 10: Whole Series including summary and remarks

　 　 Cartoon Series 1: Mechanism of viscosity
　 　 　 　 　 Series 2: Mechanism of loss generation
　 　 　 　 　 Series 3: Commuter packer and enthalpy
　 　 　 　 　 Series 4: Skinny grasshopper and fat pupa
　 　 　 　 　 Series 5: From hand-drawn picture to ready-made clip art
　 　 　 　 　 Series 6: Why do you learn? - 1
　 　 　 　 　 Series 7: Why do you learn? - 2
　 　 　 　 　 Series 8: Two Sciences
　 　 　 　 　 Series 9: Scientist and Engineer
　 　 　 　 　 Series 10: Science and Non-science
　　　　　 　 Series 11: Structure of human relations
　　　 　 　　Series 12: Oriental subconsciousness

　 　 Nostalgia Series 1: The first own car
　 　 　 　 　 Series 2: The first driver's license
　 　 　 　 　 Series 3: Comparison of German and Japanese Diploma
　 　 　 　 　 Series 4: The first air travel
　 　 　 　 　 Series 5: Tachometer used 60 years ago
　 　 　 　 　 Series 6: The three sacred calculators
　 　 　 　 　 Series 7: The first home computer
　 　 　 　 　 Series 8: Sentimental journey to Nordkapp
　 　 　 　 　 Series 9: Change of photograph
　 　 　 　 　 Series 10: Story of passport

　 　 The others