The valve is enabled, by being endowed with the force of stored energy, to generate an opening/closing motion with a small operating torque infallibly and stably, permit a reduction in cost, allow a compact construction, excel in durability, and enjoy a highly satisfactory practical value. Further by endowing the valve body which has already acquired the function of water stop with the function of steady flow volume thereby diversifying functionality, the valve is enabled to acquire the function of varying steady flow volume by virtue of the rotational frequency of the spiral spring. In the type of spring for opening/closing and controlling the flow path for a fluid, a leaf spring valve is provided which has one end of a spiral spring 47 mounted in a valve chamber 42 of a valve box 41 and the other end thereof mounted on a valve shaft 52 in the valve chamber 42, and the lateral surface of the spiral spring 47 opposed in a pressed state to a valve seat 42a of the valve box 41, whereby the lateral surface of the spiral spring 47 is separated from the valve seat 42a till a flow path outlet 44 on the secondary side is opened by rotating the valve shaft 52 by means of an actuator 55 thereby displacing the spiral spring 47 centripetally toward the valve shaft 52.