PURPOSE: To prevent condensate from forming in the exhaust pipe by preventing the exhaust gas temperature from becoming reduced even at low burning intensity disposing a shutter in a part of a passage in a secondary heat exchanger so that part of the passage in the secondary heat exchanger may be closed off under a certain temperature.

CONSTITUTION: A secondary heat exchanger 4 has a main passage 4a and a bypass 4b, and a shutter 6 and a spring 7 to open or close the shutter 6 are disposed at the outlet of the bypass passage 4b. The spring 7 is made of shape memory alloy, and close off the bypass passage 4b by pushing the shutter 6 when the allowable temperature for the exhaust gas condensation is reached while burning at low intensity. Therefore, the exhaust gas flows only through the main passage 4a and is expelled out of the exhaust gas pipe 5, resulting in the reduced heat exchanging efficiency and the increased exhaust gas temperature to prevent condensation in the exhaust gas pipe 5.