In a cogeneration apparatus, a heat exchanger includes an exhaust gas passageway for passage therethrough of exhaust gas, a heat medium passageway for passage therethrough of a heat medium that gives heat energy to a water heater etc., and a water passageway for taking heat from the exhaust gas and giving the heat to the heat medium. Cooling water for a water-cooled engine is given to the water passageway. The cogeneration apparatus requires reduced numbers of the heat exchanger and pipes, and hence a reduced number of necessary component parts, so that the cogeneration apparatus can be reduced in cost.