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.
