A fuel cell generates electric power based on electrochemical reaction of reaction gases. A gas feed stopping device stops the feed of reaction gases into the fuel cell. At the time the power generation by the fuel cell is terminated, the fuel cell continues power generation for a while under a condition that the feed of one of the reaction gases is stopped by the gas feed stopping means. With this operation, the residual reaction gas in the fuel cell is consumed smoothly while the fuel cell continues power generation after the feed of one reaction gas is stopped. The electric potential of a cathode electrode decreases promptly. Deterioration of the cathode electrode is surely prevented.