In a bellows mechanical seal, a collar attached to a seal ring through a bellows has a restriction piece extending toward at least one of an inner diameter side or an outer diameter side. The bellows mechanical seal includes an adaptor which is attached to the collar, whose axial movement is restricted by the restriction piece, and which extends to the inner diameter side or the outer diameter side over the restriction piece. The collar and the adaptor form at least part of a housing space (S), (S′) having the secondary seal housed therein.