To provide a control technique during system power supply failure of a cogeneration system incorporating a power storage device.

While a system power source 6 is in normal energization, operation of a power generation unit 2 is suspended temporarily when a hot-water storage tank has reached a full storage state. In no full storage state (when the temperature of the stored hot water has dropped), the operation of the power generation unit 2 is resumed. When power failure is detected by a power failure detection device 4b during operation, power supply interruption devices 4b and 4c are suspended and reverse charge to the system power source side is prevented. Simultaneously, electric power is supplied from a storage battery side and additionally by the rated output operation of the power generation unit 2. Thereby, when a hot-water storage tank 3a is in the full storage state, a heat radiation circuit 2a side is opened and rated operation is continued through heat radiation promotion. When the hot-water storage tank 3a is in no full storage state, the heat radiation circuit 2a side is closed.