An adaptor 6 for an endoscope includes: an endoscope attachment portion 13； a duct body 21 that communicates with a treatment instrument insertion port of the endoscope； a duct member 22； a water feeding duct cover 31 including an insulating member, the water feeding duct cover covering at least a part of an outer surface of the duct member 22 and including an extension portion 35 that spreads toward a proximal end side； and a duct body cover 41 including an insulating member, the duct body cover covering at least a part of an outer surface of the duct body 21, and covering the extension portion 35 of the water feeding duct cover 31 so as to prevent the water feeding duct cover 31 from moving to a distal end side of the duct member 22 and coming off.
