The main body of the heat accumulation element has a honeycomb structure, and fluid passages where a fluid circulates and heat accumulating medium portions where a medium for storing heat is enclosed are formed. Specifically, the heat accumulation element has partition walls, and one opening portion and the other opening portion of each of the predetermined cells of a honeycomb structure having a large number of cells partitioned and formed to function as fluid passages are plugged and fired to form plugged cells. A heat accumulating medium is provided in the plugged cells. Open cells neither plugged nor fired serve as fluid passages with the plugged cells plugged and fired serving as heat accumulating medium portions, and the fluid circulating through the open cells and the heat accumulating medium in the plugged cells exchange heat.