Wave-like concaves/convexes are formed on respective flat surfaces of a heat exchange tube, such that crest lines of continuing V shapes in a horizontal direction and trough lines of continuing V shapes in the horizontal direction are alternately arranged and that bends of the V shapes form curve lines. This configuration reduces the stress concentration at the bends in a press-forming process, compared with a configuration that crest lines and trough lines have bends formed in a shape of continuing sharply-angled V shapes in the horizontal direction. As a result, this configuration increases the yield in the press-forming process at a fixed amplitude of the wave-like concaves/convexes and increases the amplitude of the wave-like concaves/convexes at a fixed yield in the press-forming process.