<p>Two types of cells (first cells and second cells) are used to constitute a honeycomb structure. The first cells and second cells differ in the catalyst supporting position. The first cells and second cells are alternately arranged. The catalyst supporting position of the second cells is shifted in the direction of the downstream side of the flow of an air-fuel mixture from the catalyst supporting position of the first cells so that when an exothermic reaction occurs on the second cell side of a partition wall for separating a first cell from a second cell, an endothermic reaction occurs on the opposing first cell side of the partition wall.</p>
