Please note that this might not be the latest information, since I haven't been there for a few months.
According to the website, Northern Boobook (アオバズク in Japanese) has moved to the raptor area after the closure of the bird house.It is on exhibit there from May 27th.
【6/20更新】バードハウスとバードケージの鳥類展示状況
It could be on exhibit along with other raptors, although it could be more difficult to photograph due to cage and far distance. The area is located on the right hand side if you enter from the main gate. It shows Snowy Owl on the map.
Hope you see it!