A cogeneration system that is operated in system connection is disconnected from the system and operated independently at a power failure of the system. A power generator 1 driven by an engine E is connected to a system 9. A water-cooling device 13 recovers exhaust heat of the engine E. When an operating switch 20 is switched, to its independent operation mode for an independent operation apart from the system 9, the upper limit of the target rotation speed Ntgt of an electronic governor 16 is increased over that at system connection operation, thereby the engine speed can be increased, and the maximum output point of the power generator 1 is increased for operation. When a power failure of the system 9 is detected by a power failure detecting unit 21, an operation mode judging unit 19 outputs a judgment result similar to that at the moment when the independent operation mode is instructed and increases the upper limit of the target engine speed Ntgt, and turns off a connection switch 7 to disconnect the system connection.