PURPOSE: To facilitate the economical operation of cogeneration system.

CONSTITUTION: A commercial power source 1 and the electric output 3a of a cogeneration system 2 are changed over through a changeover apparatus 5, and a water passing passage 7 between the heat exchanger 8 of the cogeneration system 2 and the heat exchanger 10 of a hot water boiler 9 is changed over through a change-over valve 11 to supply an electric load 6 and a thermal load 12. The load consuming quantity Wd of the electric load 6 is detected by an electric load detector 13, and the fuel flow rates Ff of the cogeneration system 2 and of the hot water boiler 9 are detected by a fuel flow rate detector 14, and an electric power rate and a fuel charge are severally converted into commercial electric power and computed in a computing section 15. Only in the case where the electric power rate is higher than the fuel charge, the cogeneration system 2 is automatically operated, and moreover the change- over apparatus 5 and the change-over valve 11 are changed over toward a generator 3 and the heat exchanger 8 to connectingly supply the electric output 3a and a waste heat recovering hot water 2a to the electric load 5 and the thermal load 12.