An adaptor for an endoscope includes: an endoscope attachment portion； a duct body that communicates with a treatment instrument insertion port of the endoscope； a duct member； a water feeding duct cover including an insulating member, the water feeding duct cover covering at least a part of an outer surface of the duct member and including an extension portion that spreads toward a proximal end side； and a duct body cover including an insulating member, the duct body cover covering at least a part of an outer surface of the duct body, and covering the extension portion of the water feeding duct cover so as to prevent the water feeding duct cover from moving to a distal end side of the duct member and coming off.
