A fuel cell (1) comprises an anode (9A) and cathode (9B) disposed on either side of a solid polymer electrolyte membrane (8A), and power is generated by supplying hydrogen to the anode (9A) and supplying hydrogen to the cathode (9B). When the temperature of the fuel cell (1) is below 0.degree. C., water in the cell freezes. When a power plant using the fuel cell (1) is warmed up under the temperature less than 0.degree. C., the anode (9A) is connected to the positive electrode of a secondary battery (13) and the cathode (9B) is connected to the negative electrode of the secondary battery (13) to electrolyze frozen water in the cell, thereby thawing the frozen water using the heat accompanying the electrolysis.