An apparatus for controlling a suction passage in an endoscope includes a cylinder connected between an insertion pipe, on an insertion side of the endoscope, and a suction pipe, on a suction side of the endoscope. A piston is provided in the cylinder to move in an axial direction thereof to selectively connect or disconnect the insertion and suction pipes. The suction pipe, attached to the cylinder, is open at a position opposed to an outer periphery of the piston. A communication passage, provided in the piston, connects the insertion and suction pipes when the piston is in an operative position. An opening in the communication passage at the outer periphery of the piston is surrounded by an annular seal member to establish a water-tight connection between the open end of the communication passage and an open end of the suction pipe.