An adaptor for detachably connecting a surgical instrument to a robot arm of a robotic surgical system, comprising: a base body including a first surface to be attached to the robot arm and a second surface to which an attachment surface of the surgical instrument is mounted; and drive transmission members rotatably provided on the base body, wherein the second surface of the base body includes a first guide rail and a second guide rail respectively corresponding to a first guide groove and a second guide groove provided on the attachment surface of the surgical instrument, and the first and second guide rails (63a, 63b) of the second surface are configured to be inserted respectively into the first and second guide grooves (45a, 45b) of the attachment surface, and to guide the surgical instrument to be slid to a position where the drive transmission members respectively correspond to rotation members provided on the attachment surface of the surgical instrument, wherein the adaptor further comprises: a precedence guide section that protrudes from the base body along a direction parallel to a direction in which the first and second guide rails (63a, 63b) extend and that is configured to guide the surgical instrument before the first and second guide rails (63a, 63b) guide the surgical instrument, wherein the precedence guide section is configured to guide a protrusion, which protrudes from the attachment surface of the surgical instrument toward the second surface, in the direction in which the first and second guide rails (63a, 63b) extend. a base body including a first surface to be attached to the robot arm and a second surface to which an attachment surface of the surgical instrument is mounted; and drive transmission members rotatably provided on the base body, wherein the second surface of the base body includes a first guide rail and a second guide rail respectively corresponding to a first guide groove and a second guide groove provided on the attachment surface of the surgical instrument, and the first and second guide rails (63a, 63b) of the second surface are configured to be inserted respectively into the first and second guide grooves (45a, 45b) of the attachment surface, and to guide the surgical instrument to be slid to a position where the drive transmission members respectively correspond to rotation members provided on the attachment surface of the surgical instrument, wherein the adaptor further comprises: a precedence guide section that protrudes from the base body along a direction parallel to a direction in which the first and second guide rails (63a, 63b) extend and that is configured to guide the surgical instrument before the first and second guide rails (63a, 63b) guide the surgical instrument, wherein the precedence guide section is configured to guide a protrusion, which protrudes from the attachment surface of the surgical instrument toward the second surface, in the direction in which the first and second guide rails (63a, 63b) extend. a precedence guide section that protrudes from the base body along a direction parallel to a direction in which the first and second guide rails (63a, 63b) extend and that is configured to guide the surgical instrument before the first and second guide rails (63a, 63b) guide the surgical instrument, wherein the precedence guide section is configured to guide a protrusion, which protrudes from the attachment surface of the surgical instrument toward the second surface, in the direction in which the first and second guide rails (63a, 63b) extend.