Onizawa family in Tokyo Japan

Looking for Koojiroo Onizawa with the last known address of Otaku, Tokyo Japan or his sister Fujie Onizawa Sakagawa from Kotoku Tokyo Japan

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Emily, what is your question? I'm sure you're not expecting someone to pop up on this forum saying, "Oh, I know him!" Why are you looking for these people, and when did they live in Japan? If you're a relative, you could ask the city hall for their koseki.

They are family members that I’ve never met. My mother is their sister. She lost contact with them some 20 years ago. How do I get in touch with Japan’s city hall?

if you can prove the relation by an official document, probably yes.
but, do you have an idea how to prove it ?

First of all, you may want to consult to your local Japanese Embassy or Consulate.

Meanwhile, here are the contact forms for city halls in Japan:

For Ota-ku
https://www.city.ota.tokyo.jp/cgi-bin/form_enq/formmail.cgi?d=koseki
The blanks from top are
your full name
your email address
your phone number
your home address (optional)
your gender (optional)
your age range (optional)
title of your inquiry
your inquiry

Your inquiry should be written within 400 Japanese characters, which is roughly 200 English words. Click "OK" to confirm contents, and then click 送信 to send.

For Koto-ku
https://www.city.koto.lg.jp/cgi-bin/simple_faq/form.cgi
The blanks from top are
your email address
your full name
furigana of your name (optional)
your gender (optional)
your age range (optional)
your profession (optional)
your home address (optional)
your zip code (optional)
your phone number (optional)
title of your inquiry (optional)
your inquiry

Click 送信確認 to confirm contents, and then click 送信 to send.

My advice to you is to write it as a question instead of just writing "Looking for who and who." You need to write more details, otherwise no one would know how to help you. Ask something like "How could I find so and so? They are so and so. So and so lost contact with them when. We are hoping to reunite." Preferably, add greetings.

I hope it helps you.