Hydrogen gas supplied to and then purged from a fuel cell is temporarily retained in a reservoir provided in an exhaust fuel diluter, and diluted before release into atmosphere. Cathode exhaust gas discharged from a cathode of the fuel cell is mixed to dilute the hydrogen gas, and air continues to be supplied to the cathode of the fuel cell for a specific period of time even after generation of electric power in the fuel cell is stopped, so that the hydrogen gas retained in the reservoir is continuously diluted with the cathode exhaust gas and released. Air for ventilation of the fuel cell may be used instead or in addition, to dilute and release the hydrogen gas retained in the reservoir after the generation of electric power in the fuel cell is stopped.