When an ignition switch is turned off, a hydrogen supply valve is closed. Operation of a compressor continues, and an air supply valve and an air discharge valve are opened to supply the compressed air into a fuel cell through a hydrogen supply port. The scavenging process of removing the fuel gas and the water remaining in the fuel gas channel is performed through the air discharge valve, a drain valve, and a hydrogen purge valve. Then, all the valves connected to the fuel gas channel are closed. The pressure when the valves are closed, and the pressure when a short time has elapsed after closing the valves are detected by a pressure sensor. Based on the pressure difference, a failure due to the leakage in the fuel gas channel is detected.