PURPOSE: To eliminate the color and smell of exhaust gas and to reduce the amount of air pollutants in the exhaust gas.

CONSTITUTION: An oxydizing catalyst 14 to act in a high temperature range is provided in the downstream of a main combustion chamber 1, a recombustion chamber 8 and a tertiary combustion chamber 12. In this constitution, by the heat of the exhaust gas passing through these three chambers 1, 8 and 12, the catalyst 14 is heated up to a temperature high enough for reaction, and thereby the catalyst 14 oxydizes unfurnt gas in the exhaust gas.