A washing instrument comprises a washing instrument body to which a distal configuration portion of an insertion device is attached, a three-way cock which communicates with both a first washing opening and a second washing opening that spout out a washing liquid at different positions inside a receiving chamber of the distal configuration portion and thus switches communication directions and which adjusts a liquid supply volume, a liquid supply function comprising a syringe coupling portion having a syringe to supply the washing liquid and a check valve, and a movement regulating portion which positions the distal configuration portion at a position where the washing liquid is directly spouted out from the first washing opening and the second washing opening to a swing base that is provided in the receiving chamber and that swings the movement direction of the extension of a treatment instrument.