PURPOSE: To prevent corrosion or the like due to dew water by providing a dew water neutralizing device at one part of the exhaust gas path of an exhaust tube.

CONSTITUTION: The dew water neutralizing device 6 is provided at one part of the exhaust gas path of the exhaust tube 4. The dew water, generated in an exhaust pipe or the exhaust tube 4 and having the property of strong acid, is neutralized to neutral water substantially in case it passes the surface of the dew water neutralizing device 6 provided in the exhaust gas path of the exhaust tube and is dripped from an exhaust port 5. Thus, the dew water, generated due to the insufficient heat insulation of the exhaust pipe or due to the combustion air of low temperature, is neutralized, therefore, corrosion or the like due to the dew water dripped from the air feeding and exhausting device may be prevented.