To achieve miniaturization of a flapper drive source such as a torque motor, and energy saving by keeping a force for driving a flapper at a constant value regardless of a flapper position.

When a maximum sectional area of a pressure-receiving member 13 is designated as A, a force F' acting on a flapper 30 can be kept at a constant force as shown by a formula; F'=PsxA=constant. That is to say, the force F' acting on the flapper 30 can be kept constant regardless of a position of the flapper 30. When nozzles 15 and 25 are opposed to each other, the force F' acting on the flapper 30 can be made to O, provided that maximum sectional areas of the pressure receiving members 13, 23 are designated as A. That is to say, the force F' acting on the flapper 30 can be made to 0 regardless of the position of the flapper 30.