A heat exchanger comprises a plurality of heat exchange segments juxtaposed in a housing, and a plug member connected fluid-tightly to the housing, and supporting the heat exchange segments to provide a coolant or cooling medium passage in each gap between the heat exchange segments adjacent to each other. Each heat exchange segment comprises a case having an opening only on a surface of the case, at least outside of the opening being plugged fluid-tightly by the plug member, and a guide member, e.g., fin accommodated in the case, and provided with a plurality of passages allowing only gas flow in a predetermined direction, and gas intake passages and gas exhaust passages at the upstream and downstream thereof, wherein an opening of the case is provided with a gas inlet port communicated with the gas intake passage, and a gas outlet port communicated with the gas exhaust passages.