In a suction valve in which a suction conduit is connected from a treatment instrument insertion channel through a branch portion so as to open/close this suction conduit, there is no passage between a side face of a valve-portion passage in the valve and a side face of a piston shaft, but only the valve-portion passage and an inner-shaft passage of the piston shaft are used, and their diameters d3 are made larger than a diameter d2 of the suction conduit. Also, the diameter d2 of the suction conduit is made larger than an inner diameter d1 of the treatment instrument insertion channel. As a result, a suction load when a solid substance, a substance with high viscosity or the like is suctioned is reduced, and clogging by the suctioned substances in a passage in the suction valve is prevented.
