An adaptor to be provided between a drive part provided at a robot arm and including a driving member and a surgical instrument including a driven member, comprising: a drive transmission member provided to be rotatable to transmit a driving force from the driving member of the drive part to the driven member of the surgical instrument, wherein the drive transmission member includes: a first member to be fitted to the driven member of the surgical instrument; and a second member relatively movable with respect to the first member in directions toward the surgical instrument and toward the drive part, the second member being to be fitted to the driving member of the drive part, the first member and the second member are configured to be rotated integrally with each other about a rotational axis extending the directions, wherein the second member includes a projection that comes in contact with the driven member of the surgical instrument to restrict a movement of the second member toward the surgical instrument in a state where the driving member of the drive part is fitted to the second member and the driven member of the surgical instrument is fitted to the first member, the first member includes a through hole in which the projection of the second member is inserted. a drive transmission member provided to be rotatable to transmit a driving force from the driving member of the drive part to the driven member of the surgical instrument, wherein the drive transmission member includes: a first member to be fitted to the driven member of the surgical instrument; and a second member relatively movable with respect to the first member in directions toward the surgical instrument and toward the drive part, the second member being to be fitted to the driving member of the drive part, the first member and the second member are configured to be rotated integrally with each other about a rotational axis extending the directions, wherein the second member includes a projection that comes in contact with the driven member of the surgical instrument to restrict a movement of the second member toward the surgical instrument in a state where the driving member of the drive part is fitted to the second member and the driven member of the surgical instrument is fitted to the first member, the first member includes a through hole in which the projection of the second member is inserted. a first member to be fitted to the driven member of the surgical instrument; and a second member relatively movable with respect to the first member in directions toward the surgical instrument and toward the drive part, the second member being to be fitted to the driving member of the drive part, the first member and the second member are configured to be rotated integrally with each other about a rotational axis extending the directions, wherein the second member includes a projection that comes in contact with the driven member of the surgical instrument to restrict a movement of the second member toward the surgical instrument in a state where the driving member of the drive part is fitted to the second member and the driven member of the surgical instrument is fitted to the first member, the first member includes a through hole in which the projection of the second member is inserted.