To realize application to a domestic fuel cell system by preventing an inverse load flow of superfluous power to a commercial electric power system.

This concerns a system which connects a fuel cell 1 to a load 4 which is side a home and connected to a commercial power source, and supplies power to the load 4 along with the commercial power source 3. This is a controller for a domestic fuel cell system, which is provided with sensors 6, 7 arranged in a circuit for supplying power to the load 4 and detect power supplied to the load 4, an inverse load flow preventing control unit 5 for controlling the quantity of power supply from the fuel cell 1, on the basis of power detected by the sensors 6, 7 and supplied to the load 4, and a DC-to-AC converter 2 for converting into AC power, a DC power generated by fuel cell 1 which is controlled on the basis of the power detected by the sensors 6, 7 and supplied to load 4, and performs control, so that electrical energy supplied from the fuel cell 1 will not exceed the power consumption of the load 4.