Yep... it's 8:30am~5pm Monday~Friday according to Oita City's Japanese website:
http://www.city.oita.oita.jp/cgi-bin/odb-get.exe?WIT_template=AC020000&WIT_oid=icityv2::Contents::26600
(scroll down to éåºæé near the bottom of the page). By the way, that website is really crap, obviously made by a bunch of bureaucrats who want to make information as difficult to find as possible -_-
Kisetsu: I wonder why the marriage and divorce department is open 24 hours...
Wuzaracer: Erm, the English website does not provide opening hours. Did you actually READ through the website before you post the link? If you want your 2 points, don't be lazy.