In a cogeneration system having three generation units each equipped with a generator and an internal combustion engine, a single hot water tank is prepared for the three generation units to contain hot water heated by exhaust heat of the engine. The temperature of the hot water contained in the tank and a power demand of an electrical load are detected. Then, the number of the generation unit or units to be operated is determined based on the detected power demand when the detected hot water temperature is equal to or less than a first predetermined value, and operation of the generation unit or units determined to be operated is controlled, thereby reducing a space for the installment of the tank and heat loss from the tank.