Abstract: A cylindrical body of a catalyst container has that overlapped portion of a rolled sheet of metal which is welded in an axial direction of the cylindrical body. A coned portion is welded, at a large diameter portion thereof, to each axial end of the cylindrical body. The large diameter portion of the coned portion has a flange which extends radially outward, and the axial end of the cylindrical body is brought into abutment with an outward end surface of the flange and is fillet-welded along an outer corner which is formed by the flange and the cylindrical body. A heat shield cover is mounted under the casing by connecting a second flange connected to a front end of a downstream exhaust gas pipe to a first flange connected to a rear end of the casing. The heat shield cover is made up of a mounting flange portion and a cover portion. The mounting flange portion is extended so as to abut with an upstream surface of a lower part of the first flange.