A valve element movement mechanism includes: a first valve stem having one end portion to which a first valve element to close and open an area between one of the large diameter portion and the small diameter portion is fixed; a second valve stem having one end portion to which a second valve element to close and open an area between the other of the large diameter portions and the small diameter portion is fixed; a first bellows having one end portion fixed to a first bellows flange and the other end portion fixed to the first valve element; a second bellows having one end portion fixed to a second bellows flange and the other end portion fixed to the second valve element; a frame disposed outside the main passage; and an actuator to move the frame in the directions in which valve elements are closed and opened.