An air passage 2 is provided with an air passage groove 5 that supplies a required volume of air to the side downstream from the fully closed position of a valve body for control purposes. Further, a taper is formed in such a way that a predetermined volume of air can be supplied, even if positional variations have occurred to the valve body. This arrangement allows the volume of air required for the traveling of a car to be supplied, without increasing the number of parts, even if the valve body 3 has stuck downstream from the controlled area. This arrangement also permits a stable supply of air flow at the idle position of the valve body.