A heat exchanger according to the present invention includes: a pillar shaped honeycomb structure having a plurality of cells, the cells providing first flow paths through which a first fluid is passed; an inner cylinder attached to an outer periphery of the honeycomb structure; and an outer cylinder disposed on an outer periphery of the inner cylinder, the outer cylinder providing a second flow path through which a second fluid is passed, the second flow path being arranged between the outer cylinder and the inner cylinder. The second flow path includes: an intermediate flow path extending in an axial direction of the honeycomb structure so as to include an outer peripheral position of the honeycomb structure; and side flow paths located on both sides of the intermediate flow path in the axial direction. The intermediate flow path has a height lower than that of each of the side flow paths.