In a cogeneration apparatus having a power generation unit connected to an AC power feed line between a commercial power source and electric load, and an internal combustion engine, a switch is installed between the power generation unit and an outage power output, and the switch is opened to combine the power generation unit with the commercial power source to supply combined power to the electric load if a calculated total engine operation time period is not more than a predetermined time period when the outage is determined not to occur, while the switch is closed to connect the power generation unit to the outage power output, if the power generation unit is determined to be capable of generation power based on an estimated degree of deterioration of the unit when the outage is determined to occur.