In a cogeneration system having a first power plant connected to an AC power feed line between a power network and an electrical load and a first internal combustion engine for driving the first power plant such that exhaust heat of the first engine is supplied to a thermal load, power supply from the first power plant to the power network is interrupted by turning off a switch installed in the feed line, when outage of the power network is detected and a second power plant is operated, such that outputs of the first and second power plants are supplied to the electrical load. With this, it becomes possible to respond to a commercial power network outage for preventing reverse flow of the power output by the cogeneration system into the power network and supplying as much electric power as possible to the electrical load.