In a heat exchanger, a tube is adapted to exchange heat between a first fluid flowing therein and a second fluid flowing through outside of the tube, and an inner fin is disposed in the tube to divide a flow passage in the tube into a plurality of flow paths. The inner fin includes a plurality of fin portions with different specifications, and the fin portions are arranged in series with respect to a flow direction of the first fluid. Furthermore, the fin portion with the smallest flowing resistance of the first fluid among the plurality of fin portions is arranged on an upstream side of the flow direction of the first fluid with respect to at least an another fin portion. Accordingly, heat exchange performance in the entire heat exchanger can be effectively improved.