A treatment instrument insertion auxiliary is used to subserve passage of an elongated treatment instrument into a treatment instrument channel by being connected to an endoscope provided with a rigid insertion portion which includes the treatment instrument channel adapted to allow passage of the treatment instrument and a treatment instrument channel opening portion formed at a proximal end of the treatment instrument channel and used to insert the treatment instrument, where the treatment instrument insertion auxiliary includes a receiving portion adapted to suppress vibration of a distal end side of the treatment instrument by being placed in contact with the distal end side of the treatment instrument and a first guide unit adapted to guide a distal end of the treatment instrument placed in contact with the receiving portion in a central axis direction of the treatment instrument channel opening portion.
