PURPOSE: To reduce a drop in power generation characteristic due to the excessive wetting of a cathode electrode solvent layer by providing a reducing liquid feed means for feeding and keeping a reducing liquid to and with a cathode.

CONSTITUTION: When a cell output drop is observed, due to the excessive wetting of a cathode solvent layer during the operation of a phosphoric acid fuel cell, the operation is temporarily stopped at an elevated temperature. In this case, the circulation of nitrogen gases to a cathode 3b is started to purge residual oxygen therefrom. After a sufficient amount of nitrogen gases is supplied to the cathode 3b, formalin mists generated with a mist generator 23 are fed thereto, using the nitrogen gases as carrier gases, thereby reducing the cathode 3b. Also, after reducing the cathode 3b, the operation of the fuel cell is resumed.