In a valve body, an input chamber is located between a small diameter slide hole, which slidably supports a small diameter portion of a slidable valve, and a large diameter slide hole, which slidably supports a large diameter portion of the slidable valve, at a location that is closer to the small diameter slide hole than to the large diameter slide hole. An output chamber is located between the small diameter slide hole and the large diameter slide hole at a location that is closer to the large diameter slide hole than to the small diameter slide hole. A valve opening communicates between the input chamber and the output chamber and is opened and closed in response to a slide position of the slidable valve. A pilot chamber exerts a pilot pressure to urge the slidable valve in a valve closing direction toward the valve opening.