To restrain mutual communication of exhaust passages by superposing an end part of a second partition plate on a surface of a first partition plate positioned on the opposite side of the bending direction of a circular arc-shaped side edge part, and extending one side edge of the end part of the second partition plate up to an exhaust pipe inner wall surface on the circular arc-shaped side edge part side.

One side edge 80 of an end part of a cutoff plate 70 positioned on the opposite side of the bending direction of one circular arc-shaped side edge part 54, is extended up to a pipe inner wall surface 50 on the circular arc-shaped side edge part 54 side. One side edge 82 of an end part of a cutoff plate 72 positioned on the opposite side of the bending direction of the other circular arc-shaped side edge part 56, is extended up to the pipe inner wall surface 50 on the circular arc-shaped side edge 54 side. That is, the respective cutoff plates 70 and 72 have central cutoff parts 86 and 88 superposed in a sealed state on a central part 84 of a first partition plate except for circular arc-shaped side edge parts 54 and 56 and one side edges 80 and 82 positioned on the opposite side of the bending direction of the circular arc-shaped side edge parts 54 and 56 adjacent to them. Therefore, communication between exhaust passages 76 and 78 in both side edge areas of the first and the second partition plates can be cut off.