the valve housing 54 a valve hole 54 a is formed so as to constitute a part of the supply passage 39 as the valve body 51 moves downward the opening degree of the valve hole 54 a is increased in contrast as the valve body 51 moves upward the opening degree of the valve hole 54 a is reduced the pressure sensing mechanism 52 includes a pressure sensing chamber 52 a and a bellows 52 b the pressure sensing chamber 52 a is formed in an upper part of the valve housing 54 the bellows 52 b is accommodated in the pressure sensing chamber 52 a and serves as a pressure sensing member in the pressure sensing chamber 52 a the pressure at a first pressure monitoring point p 1 is introduced into the internal space of the bellows 52 b through a first pressure detecting passage 55 also in the pressure sensing chamber 52 a the pressure at a second pressure monitoring point p 2 is introduced into the external space of the bellows 52 b through a second pressure detecting passage 56 the bellows 52 b provides the valve body 51 with downward pressing force that results from differential pressure u03b4p between the first pressure monitoring point p 1 and the second pressure.