To achieve good improvement in heat exchange efficiency while ensuring that the vertical thinness of the structure remains good.

Laterally on the two sides of a pair of header gropes 4a, 4b erected each pipe in parallel with the other pipe groups of heating tube elements are provided in a pair 5a, 5b. Each group of heating tube elements 5a, 5b is comprised of a plurality of heating tube elements 8, 8, each shaped flat with a passageway turned back inside, and corrugated type fins 9, 9, the elements and fins being laid one over another in alternate layers. The two ends of the turned-back passageway are put in tight communication with the inside of the pair of header pipes 4a, 4b. Of this pair of header pipes 4a, 4b, the inside of one 4a is divided by a partition plate 18 into two, that is, an inlet chamber 19 and an outlet chamber 20.