PURPOSE: To prevent the heating of the upstream side of a main catalyst body by disposing a catalyst body having a further smaller opening area at the upstream side of the main catalyst body.

CONSTITUTION: The thickness of an upstream side catalyst 7 is decreased, and the size of each of small holes 8 is made smaller than that of the main catalyst body 6, and only a part of the upstream surface 9 or the downstream side surface 10 is blocked. By this procedure, it is sufficient even if the heat capacity of the catalyst body 7 is not made large, and it is also possible to curtail the time of heating the catalyst body 7 and the catalyst body 6. Further, back fire from the back side of the catalyst body 7 to the upstream side can be prevented.