A heat exchanger includes a plurality of fins that are spaced from each other and are arranged to divide a heat medium flow passage into a plurality of narrow passages. Each fin incudes a plurality of thick wall portions and a plurality of thin wall portions which are alternately arranged in a passage longitudinal direction. Each adjacent two of the plurality of fins, which are adjacent to each other, are defined as one fine and another fin, and each of the plurality of thick wall portions of the one fin is opposed to an adjacent one of the plurality of thin wall portions of the another fin in a fin arrangement direction, and each of the plurality of thin wall portions of the one fin is opposed to an adjacent one of the plurality of thick wall portions of the another fin in the fin arrangement direction.