A heat exchanger assembled from multiple heat exchanging tubes. Each of the multiple heat exchanging tubes is formed as a flattened tube of 0.5 mm in thickness by press work or bending work of a stainless steel plate member having a thickness of 0.1 mm. Each of the multiple heat exchanging tubes is structured to have multiple lines of sequential wave crests and multiple lines of sequential wave troughs formed on each of flattened faces of the heat exchanging tube. The multiple lines of the sequential wave crests and the multiple lines of the sequential wave troughs are arranged to have a preset angle γ relative to a main stream of an air flow. The lines of the sequential wave crests and the lines of the sequential wave troughs are symmetrically folded back about folding lines arranged at a preset interval W along the main stream of the air flow.