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main building (as opposed to an annex, outbuilding, etc.)
this library; this museum; this theatre; this hall; this building
美術館の本館と別館を結ぶ屋根付きの通路がある。
There is a passage connecting the main building and annex.
総合受付は本館の一階ロビーの正面にあります。
The reception desk is on the first floor of the main building.
老朽化した本館の大規模な改装工事が来月始まる予定だ。
Renovation work on the main building has begun.