An adapter for detachably connecting a surgical instrument to a robot arm of a robotic surgical system according to an embodiment may include: a base including a first surface to be attached to an attachment portion of the robot arm, a second surface, and a plurality of recesses formed in the first surface； an engagement member disposed in the base and movable between an advanced position where the engagement member is advanced into the plurality of recesses and a retracted position where the engagement member is retracted from inside the plurality of recesses； a biasing member which biases the engagement member in a direction from the retracted position to the advanced position； and an operating member to be operated to move the engagement member to the retracted position against a biasing force of the biasing member.
