A purge solenoid valve is opened to introduce an atmospheric pressure into a fuel tank, the purge solenoid valve is closed to operate a negative pressure pump, and set an internal pressure of the tank to a reference pressure Pb. Then, a changeover valve is closed, and the purge solenoid valve is opened to cause a canister to communicate with an intake passage. Further, the changeover valve is opened, and the purge solenoid valve is closed. Then, a first tank internal pressure Po is detected and subtracted from the reference pressure Pb. If a calculated first pressure deviation ΔPo is larger than a first threshold ΔP1, it is determined that there is no open sticking in a sealing valve.