A fixing sleeve that is fitted outward on a connecting portion between a connecting pipe and a treatment tool inserting tube for fixation of the connecting portion comprises a cylindrical sleeve body, an arc-shaped portion that is connected to a proximal end side of the sleeve body, is formed to be coaxial with a sleeve center axis of the sleeve body and has a center angle smaller than 180°, and a projection portion that is formed along a peripheral direction of the sleeve center axis on an inner peripheral surface in the arc-shaped portion in a radial inside of the sleeve center axis. A part of the treatment tool inserting tube is flexibly deformed in a convex shape by the projection portion, and the part thereof is supported by the projection portion.
