In a cogeneration apparatus having a power generation unit equipped with a power generator, an internal combustion engine, and a hot water tank connected to the engine through a heat exchanger, there are provided a first pump provided at a flow channel connecting the engine to the heat exchanger and a second pump provided at a second flow channel connecting the heat exchanger to the hot water tank. A temperature of the engine cooling water heated by the engine and a temperature difference between the temperature and a temperature of the engine cooling water cooled by the heat exchanger is determined and based on the determined values, flow rates of the first and second pumps are controlled.