PURPOSE: To eliminate an effect of an opening pressure of a check valve disposed inside a vaporized fuel supply system.

CONSTITUTION: A casing 11 is divided into two chambers, i.e., a pressure chamber 15 and a negative pressure chamber 17 via a diaphragm 13 so that an FCV 10 is disposed in a vaporized fuel pipeline communicating a fuel tank with a canister. A return spring 25 for energizing the diaphragm 13 in such a direction as to close the communication of an inlet port 21 with an opening port 23 is contained in the negative pressure chamber 17, and is connected onto an atmosphere releasing side of the canister, to which a predetermined negative pressure is applied at the time of diagnosis.