If a pressure deviation ΔP (amount of pressure decrease) after a first predetermined time period T1 has elapsed since a bypass valve (canister opening/closing valve) is controlled from an opened to closed state by actuating a negative pressure pump when the engine is stopped and a purge valve is opened is less than a first predetermined pressure P1, it is determined that the bypass valve is in an open sticking state, and if the pressure deviation ΔP is not less than the first predetermined pressure P1, it is determined that the bypass valve is not in an open sticking state. Moreover, a pressure deviation ΔP after a second predetermined time T2 has elapsed since the bypass valve is subsequently controlled to be opened decreases by not less than a second predetermined pressure P2, it is determined that the bypass valve 37 is not in a closed sticking state.