A parallel movement mechanism including guide grooves and guide rollers and a perpendicular movement mechanism including cam grooves and cam rollers are interposed between a drive rod of an air cylinder and a valve assembly. By moving the valve assembly parallel to valve seat surfaces formed around a pair of openings of a valve casing with the parallel movement mechanism, and then moving it perpendicularly to the valve seat surfaces with the perpendicular movement mechanism, valve seal members of valve elements can be brought into and out of contact with the valve seat surfaces, and the openings can be selectively opened and closed.