Water of low temperature from a feed pump (15) is divisionally fed to an auxiliary evaporator (17) covering an exhaust port (16) extending from the combustion chamber of an internal combustion engine (E) and to a main evaporator (11) located downstream of the exhaust port (16). The direction of water flowing through the auxiliary evaporator (17) is parallel with the direction of exhaust gas flow, whereby the upstream portion of the exhaust port (16), which tends to be high in temperature, is effectively cooled by water of lower temperature, thus inhibiting the heat from escaping from the upstream portion of the exhaust port (16). The direction of water flowing through the main evaporator (11) is opposed to the direction of exhaust gas flow, thereby securing the temperature difference between water and exhaust gases throughout the length of the water passageway of the main evaporator (11), and increasing the heat exchange efficiency.
