PURPOSE: To obtain an apparatus having high and stable gas-electricity exchangeability for domestic use by providing means of holding the fuel battery output to a set value by operating fuel control means using signals given from a detector.

CONSTITUTION: The fuel gas is supplied to the fuel battery 5 from the gas supplying inlet 1 by the gas tube 2 the gas governer 3, and the ratio control valve 4, and it is converted to DC power and taken out from the output terminal 7. When load is increased and the output voltage is lowered, the output detector 8 detects it instantly and sends a signal to the control circuit 9. The control circuit 9 issues the current which is proportional to the voltage change, to the ratio control valve 4, to increase its opening and maintains the output voltage constant. An infinitesimal calculus circuit is provided in the control circuit 9 to prevent the time delay between them.