A surgical instrument according to an embodiment may include: a base body including an attachment surface to be attached to the adaptor; an elongated shaft including one end connected to the base body; a treatment tool provided on a side of the other end of the shaft, elongate elements for operating the surgical tool, driven members rotatably provided on the base body and connected with end portions of the elongate elements; a holding member provided such that one end of each driven member is rotatably held by the base body and the other end of each driven member is rotatably held by the holding member; and a movable member provided movable with respect to the holding member and the base body and engageable with an adaptor. The movable member is configured, when moved with respect to the holding member and the base body, to be disengaged from the adaptor.