Method of operating phosphoric acid fuel cell

Before a phosphoric acid fuel cell generates electric energy, a cathode is supplied with a hydrogen gas (fuel gas) and air (oxygen-containing gas) to cause a reaction between hydrogen and oxygen. At this time, the hydrogen and the oxygen are caused to react with each other under a condition in which the concentration of phosphoric acid at the time the amount of water produced by the reaction (liquid water) for reducing the concentration of phosphoric acid and the amount of water evaporated from phosphoric acid are in equilibrium is equal to or greater than a reference phosphoric acid concentration which is capable of keeping the fuel cell at a desired performance level. The temperature of an electrolyte electrode assembly of the fuel cell is increased by the heat which is generated by the reaction.