Each fin 30 is designed to have continuous lines of wave crests 34 and continuous lines of wave troughs 36 arranged at a preset angle in a specific angle range of 10 degrees to 60 degrees relative to the main stream of the air flow and symmetrically folded back about folding lines of a preset folding interval W along the main stream of the air flow. A ratio (a/p) of an amplitude ‘a’ of a waveform including one wave crest 34 and one adjacent wave trough 36 to a fin pitch ‘p’ satisfies a relation of 1.3×Re−0.5