Disclosed related to a co-generation comprising a heat exchanger 130 heating water; a heat transfer path 170 connected with the water heat exchanger130 to transfer heat; a hot water storage tank 80 connected with the water heat exchanger 130 and the water circulation path 81; a hot water storage water supply apparatus 82 supplying water in the hot water storage tank 80 to the water circulation path 819 a water supply path 83 connected with the water circulation path 81; a water supply apparatus 86 supplying water to the water supply path 83; and a hot water supply heat exchanger bypassing apparatus 87 and 88 bypassing the water supplied to the water circulation path 81 from the hot water storage tank 80 through the hot water supply heat exchanger 130. The co-generation of the present invention has some advantage in that the effective water boil heat is increased as supplying the cold water supplied from the water service and the hot water supplied from the hot water storage tank to the hot water supply heat exchanger 70 properly and heat exchanging with the heat medium such as an anti freezing solution and etc passing through the hot water supply heat exchanger.