Refers to the principal building of a complex, as opposed to an annex (別館), outbuilding, or detached wing. Common in institutional contexts such as museums, libraries, hotels, and hospitals.
The museum's main building is under renovation, so only the annex can be visited.
ホテルの本館と新館は地下通路でつながっている。
The hotel's main building and the new wing are connected by an underground passage.
this building (library, museum, theatre, hall)
Used within an institution to refer to the building itself, similar to 'this library' or 'this museum' in English. The specific meaning depends on the type of facility.
本館では、毎週土曜日に映画上映会を行っています。
This building holds a film screening every Saturday.
ご不明な点は本館１階の受付までお尋ねください。
If you have any questions, please ask at the reception desk on the first floor of this building.
The main building of the museum was constructed in the Meiji era.
The seminar will be held on the third floor of the main building.
After checking in, please proceed to the main building.