A sealing valve 28 that controls a communication state between a fuel tank
10 and a canister 26 is provided. During stop of an internal combustion
engine, the sealing valve 28 is generally closed, and the canister 26 is
opened to the atmosphere. The sealing valve 28 is opened when the
internal combustion engine is stopped, and differential pressure
exceeding a valve opening determination value is generated between tank
internal pressure and atmospheric pressure. A change in the tank internal
pressure generated between before and after the sealing valve 28 is
opened is detected. When the change in the tank internal pressure is
below a predetermined determination value, closing failure of the sealing
valve is determined.