Abstract: In a fuel-cells system 20, cooling water circulating through a cooling water flow path 28 is heated by heat exchange carried out in fuel cells 22, subsequently cooled down by a radiator 26, and fed again to the fuel cells 22. A cooling water pump 30 gives a driving force used for circulating the cooling water through the cooling water flow path 28. A control unit 50 of the fuel-cells system 20 measures an inlet cooling water temperature T1 and an outlet cooling water temperature T2 of the fuel cells 22 and calculates a difference .DELTA.T between the observed temperatures T1 and T2. When the difference .DELTA.T is not less than a reference value set equal to 5.degree. C., the control unit 50 determines that a distribution of internal temperature of the fuel cells 22 is in a predetermined non-uniform state. The control unit 50 then increases the driving voltage of the cooling water pump 30 to equalize the internal temperature of the fuel cells 22.