A decrease in voltage in a polymer electrolyte fuel cell comprising stack of unit cells caused by the temperature difference between the cells located at the ends and the other cells due to a differential in heat dissipation from end plates is prevented by controlling the cooling temperature of the cells closest to the end plates of the fuel cell without affecting the output voltage of the cells in the middle by not including a coolant flow channel in the conductive separator plate between at least one of the end plates and the unit cell located closest to the one of the end plates.