A sealing valve is installed between a fuel tank and a canister. A purge VSV is installed between the canister and an intake path. A pump module unit is installed to introduce a negative pressure into the canister. The negative pressure is introduced into the canister while the purge VSV and sealing valve are closed. An open failure diagnostic check is conducted on the sealing valve in accordance with the prevalent canister side pressure. A close failure diagnostic check is conducted on the sealing valve by making use of a differential pressure that is generated across the sealing valve as a result of the open failure diagnostic check.