An electric motor (5) and an accessory circuit (7, 8) are connected in parallel to an output terminal (1A) of a fuel cell power plant (2). A secondary battery (3) is connected to the output terminal (1A) through a DC/DC converter (6) which controls an output voltage in both directions. The accessories (7, 8) are directly driven by the output of the fuel cell power plant (2). A reduction in the output voltage of the fuel cell power plant (2) resulting from an increase in the load of the electric motor (5) is prevented by discharging power from the secondary battery (3) via the DC/DC converter (6). By directly supplying electric current output from the fuel cell plant to the accessory circuit (7, 8) without passing through the DC/DC converter (6), the power loss due to the DC/DC converter (6) is reduced.