A cogeneration apparatus having a power generation unit equipped with a power generator adapted to be connected to an AC power feed line at a location between a commercial power source and an electric load and an internal combustion engine for driving the generator, a switch installed at a location between the power generation unit and an outage power output to be connected to the electric load at outage of the commercial power source; characterized by: a total engine operation time period calculator (44a, S14, S104, S204) that calculates a total operation time period of the engine; an outage occurrence determiner (44a, S12, S102, S202) that determines whether the outage of the commercial power source occurs; a cooperative power generation controller (44a, S14 - S20, S104 - S110, S204 - S210) that controls the switch to disconnect the outage power output from the power generation unit and to connect the commercial power source and the power generation unit in order to supply combined power of the power generation unit with the commercial power source to the electric load via a distribution board, if the calculated total operation time period of the engine is equal to or smaller than a predetermined first time period when the outage is determined not to occur; a power generation capability determiner (44a, S22, S112, S114, S214 - S224) that estimates a degree of deterioration of the power generation unit and determines whether the power generation unit is capable of generating power based on the estimated degree of deterioration of the power generation unit when the outage is determined to occur; and an outage power generation controller (44a, S24 - S26, S116 - S118, S228 - S230) that closes the switch to connect the power generation unit to the outage power output to supply the power generated by the power generation unit to the electric load via the outage power output, if the power generation unit is determined to be capable of generation power when the outage is determined to occur. a total engine operation time period calculator (44a, S14, S104, S204) that calculates a total operation time period of the engine; an outage occurrence determiner (44a, S12, S102, S202) that determines whether the outage of the commercial power source occurs; a cooperative power generation controller (44a, S14 - S20, S104 - S110, S204 - S210) that controls the switch to disconnect the outage power output from the power generation unit and to connect the commercial power source and the power generation unit in order to supply combined power of the power generation unit with the commercial power source to the electric load via a distribution board, if the calculated total operation time period of the engine is equal to or smaller than a predetermined first time period when the outage is determined not to occur; a power generation capability determiner (44a, S22, S112, S114, S214 - S224) that estimates a degree of deterioration of the power generation unit and determines whether the power generation unit is capable of generating power based on the estimated degree of deterioration of the power generation unit when the outage is determined to occur; and an outage power generation controller (44a, S24 - S26, S116 - S118, S228 - S230) that closes the switch to connect the power generation unit to the outage power output to supply the power generated by the power generation unit to the electric load via the outage power output, if the power generation unit is determined to be capable of generation power when the outage is determined to occur.