A heat exchanger includes: a pillar shaped honeycomb; an inner cylindrical member; an outer cylindrical member arranged on a radially outer side of the inner cylindrical member such that a part of the outer cylindrical member forms a flow path for a second fluid; an upstream cylindrical member having a cylindrical portion and a flange portion, the upstream cylindrical member being located on a side of a first end face of the honeycomb structure, and an end portion of the flange portion being connected to the inner cylindrical member and/or the outer cylindrical member; and a downstream cylindrical member having a cylindrical portion and a flange portion, the downstream cylindrical member being located on a side of a second end face of the honeycomb structure, and an end portion of the flange portion being connected to the inner cylindrical member and/or the outer cylindrical member.