PURPOSE: To cause smooth movement of a displacement means by means of a low force by providing the displacement means at its interior with a communicating hole for intercommunicating a second fluid passage and a part of a control chamber.

CONSTITUTION: When a water amount is throttled to a lowermost value by means of a valve body 4, a first valve seat 37 of a housing 3 is brought into slide contact with a first valve part 41 of the valve body 4 to partition a first fluid passage 31 from a second fluid passage 32. In this case, an internal pressure P1 in the passage 31 is exerted rightwardly by means of a valve part 41 of the valve body 4 and leftwardly by means of a second valve part 42. Thereby, since loads having the same magnitude as each other in a reverse direction to each other are offset with each other, no load in en axial direction is generated. In relation to an internal pressure P2 in the passage 32, since a chamber 30b is communicated with the passage 32 through a communicating hole 44 formed in a valve rod 43, internal pressures in the passages 32 on the both sides in the axial direction of the valve body 4 are made equal to an internal pressure in the chamber 30b. Thereby, since a load in in axial direction exerted on the side on the passage 32 side of the valve part 41 of the valve body 4 is offset with that exerted on the side on the chamber 30b side of the valve part 42, a load in an axial direction exerted on the valve body 4 is reduced to almost zero.