Abstract: The adapter includes: a base body that includes a first surface attached to a robot arm, and a second surface to which a surgical instrument is attached; an engaging portion that engages with the robot arm at an advanced position corresponding to an opening of the first surface; an urging member that urges the engaging portion in a direction from a retracted position to the advanced position; an operation portion connected to the engaging portion to shift the engaging portion to the retracted position while resisting an urging force of the urging member; and a stopper that stops the operation portion to prevent a shift of the engaging portion from the retracted position to the advanced position.