A patient-side apparatus according an embodiment may include a robot arm; an adaptor that is attached to the robot arm; and a surgical instrument that is attached to the adaptor by sliding the surgical instrument to the adaptor. The adaptor includes: an adaptor base including an arm attachment surface and a surgical instrument attachment surface; a drive transmission member provided being movable in a direction perpendicular to the surgical instrument attachment surface and configured to transmit a driving force from the robot arm to the surgical instrument. The surgical instrument includes a surgical instrument base including an adaptor attachment surface. The surgical instrument base includes an inclined surface configured, upon slide attachment of the surgical instrument to the adaptor, to come in contact with the drive transmission member to move the drive transmission member toward the robot arm side in a direction perpendicular to the surgical instrument attachment surface.