A control apparatus and a control method are used in a fuel cell system, in which fuel gas and oxidant gas are supplied to a fuel cell stack 1 to generate electric power, a load unit 9 is driven by supplying the generated power thereto, and the generated power is stored in a power storage unit. A control unit 10 sets in advance a margin load power to be supplied from a power storage unit 8 to the fuel cell stack 1 when the load of the load unit 9 changes at a predetermined rate, and computes outputable power of the power storage unit 8. Then, the control unit 10 compares the margin load power and the outputable power to generate an amount judgment result. When the outputable power is larger than the margin load power, the control unit 10 controls the power generation amount of the fuel cell stack 1 such that charged power becomes equal to an electric power difference between the margin load power and the outputable power.