Today, Japanese WordPress develop members announced they brought the new Japanese version WordPress for us, according to the Codex for Version 3.6, there are tons of changes.
My WordPress is a multisite type and the parent site language is English. Probably for this reason, the WordPress Updates page does not show me the Japanese version update message.
So, I downloaded wordpress-3.6-ja.zip manually and upgraded.
Happy upgrade!! But, to use the new features I should update my themes I made by myself. Sigh!