To suppress cracks due to erosion or thermal stress and reduce the combustion residue of PMs at an outer peripheral part.

The cells in the outer peripheral part are formed of a material with higher thermal conductivity than that of the cells in the inner peripheral part. The heat accumulated in the inner peripheral part being transmitted to the outer peripheral part causes the temperature of the outer peripheral part to rise thereby to reduce the combustion residue of the PMs in the outer peripheral part. Further, the heat generated when the PMs are burnt can be effectively released outside and the generation of breakage or cracks due to erosion or thermal stress can be suppressed.