This treatment instrument adaptor allows a treatment instrument to be rotated about the longitudinal axis while an insertion portion is placed in the body. This treatment instrument adaptor has a cap member attached to a distal end of an insertion portion having a channel and that has a through hole aligned with an opening of the channel； a tubular fitting member attached to the through hole of the cap member so as to be rotatable about an axis of the through hole and so as not to be movable along the axis and that has an inner hole into which a treatment instrument is fitted and with which the treatment instrument is engaged in a circumferential direction； and a rotating mechanism including a driving-force transmitting member that is wound around the fitting member and that rotates the fitting member about the axis using tension.
