Pressure detection means detects the pressure of an evaporation purge system that includes a fuel tank and a communicating tube, which connects a canister for adsorbing and retaining fuel vapor to an intake pipe. Shutout means shuts out the evaporation purge system from atmospheric air. A pump is driven for pressurization or depressurization purposes while the shutout means shuts out the evaporation purge system from atmospheric air. The pump is stopped when the pressure detected by the pressure detection means reaches a predetermined level. A leak judgment is formulated in accordance with pump drive time and a pressure change after pump stoppage.