A cogeneration system including an engine, which drives a generator to generate electricity, a cooling/heating unit, which comprises at least one compressor, a four-way valve, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger, to establish a heat pump type refrigerant cycle, a pre-heat supplier to supply heat of cooling water used to cool the engine and heat of exhaust gas discharged from the engine to the outdoor heat exchanger, and thus, to pre-heat air passing around the outdoor heat exchanger, and a refrigerant over-heater to supply the heat of the exhaust gas discharged from the engine to an inlet of the indoor heat exchanger of the cooling/heating unit. In accordance with the cogeneration system, it is possible to maximize engine waste heat utilization, and thus, to achieve an enhancement in heating performance.