I'm sorry for your loss. I hope you can find your cat.
So, are you from Ibaraki?
Because the animal control center caught a male white cat on 08/18. Here's their page: http://www.pref.ibaraki.jp/hokenfukushi/doshise/hogo/documents/kouzizyouhou0818.pdf
P.S: sorry for writting in English, I thought it would be so much easier to write you a clear message.