An adaptor according to one or more embodiments may include a drive transmission member to transmit a driving force from a driving member of a drive part to a driven member of a surgical instrument. The drive transmission member may include: 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 and to be fitted to the driving member. The second member may include a movement restriction part that comes in contact with the driven member 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.