An endoscope includes an operation unit, an elongated tube, a distal opening, a suction channel, and a discharge conduit. A suction button assembly has a valve structure for changing over between the discharge conduit and the suction channel for communication and shut-off. The suction button assembly includes a plunger, a first stem end, and a valve opening of the plunger. A cap device is coupled to the first stem end and to the first cylinder end of a cylinder housing in the operation unit, and around the first stem end. A first regulating device prevents the plunger from rotating in the cap device. A second regulating device prevents the cap device from rotating about an axis of the cylinder housing, to keep the valve opening aligned with a flow opening of a cylinder passage of the cylinder housing.
