PURPOSE: To increase operation life of a fuel cell by intermittently exchanging reaction gases supplied to each of electrodes by exhausting reaction gases in gas flow lines with inactive gas.

CONSTITUTION: After a fuel cell is operated for a specified time, a fuel gas supply valve 10a and an oxidizing gas supply valve 11a are closed to stop gas supply. Inactive gas supply valve 12a and 12b are opened and inactive gas, for example, nitrogen is supplied to pipe lines and a fuel cell 5 to exhaust reaction gases in arrow directions B and D. Then valves 12a and 12b are closed, the valve 10a is opened, the valve 10b is closed, the valve 10c is opened. The valve 11a is opened, the valve 11b is closed, the valve 11c is opened. When reaction gases are supplied, former anode operates as a cathode, and former cathode operates as an anode and electricity having opposite polarity is generated in electrodes M and N and supplied to a load after changing polarity with a polarity exchanger 12.