A heat exchanger, including: a columnar honeycomb structure having cells partitioned by partition walls composed of ceramics, each cell penetrating from a first end to a second end to form a flow path for a first fluid; an inner tubular member fitted to an outer peripheral surface of the honeycomb structure to circumferentially cover that surface; a spacer directly and circumferentially covering an outer peripheral surface of the inner tubular member as well as indirectly and circumferentially covering the outer peripheral surface of the honeycomb structure; and an outer tubular member directly and circumferentially covering the spacer; wherein the spacer has a three-dimensional structure that allows flow of a second fluid therein and suppresses movement of bubbles in the second fluid; and wherein at least one opening part between the inner tubular member and the outer tubular member forms a gate of the spacer for the second fluid.