A medical treatment instrument operation unit includes a sliding portion capable of advancing and retracting to a body portion and rotating around a longitudinal axis of the body portion and connected to a proximal end portion of the operating member, and a regulating portion switchable to a movable state where regulating portion is capable of advancing, retracting, and rotating to the body portion and a limited state where the advance, retraction, and rotation of sliding portion to the body portion is limited. The regulating portion includes a rotatable dial portion, and a shaft-shaped member screwed to the dial portion to be relatively advanceable and retractable thereto in a direction in which the shaft-shaped member is brought close to and separated from the body portion. Movable and limited states are switched as the shaft-shaped member comes close to and separates from the body portion with the turning of the dial portion.