The present invention provides a highly efficient fuel cell power supply unit, which is constructed by directly connecting a fuel cell with a capacitor. In this power supply unit, a control device of the unit calculates the output voltage V2 of the fuel cell after the variation of electrical load based on the synthetic current-voltage characteristics of the fuel cell and the capacitor and the predetermined width of the variation of electrical load ΔI, calculates the corresponding current Ifc2′, and then calculates the equilibrium reacting gas supply amount Qa1, and supplies an excess amount of the reacting gas exceeding Qa1 before the variation of electrical load.