PURPOSE: To provide the outer boundary film of heat transmission pipe with an increased coefficient of heat transfer for thereby improving its heat transfer, by improving the opening of buffle plate in its configuration and heat transmission pipes in their arraying structures.

CONSTITUTION: A buffle plate 11 has an opening 34 formed at the center thereof for insertion of inner cylinder 5, and a through-hole 35 for the passage of heat transfer pipe 8 therethrough. The through-hole 35 is formed slightly larger in diameter than the outer diameter of heat transmission pipe 8 including a spiral fin 13, so that the heat transmission pipe 8 may be lightly engaged into the hole 35. The buffle plate 11 is inserted into and welded to an inner cylinder 5, engaging lightly with an outer cylinder 9. The spiral fin 13 of heat transfer pipe 8 is secured to the outer peripheral surface of heat transfer pipe 8 by a high frequency welding, but is not provided at the upper portion of heat transfer pipe 8, i.e., above the uppermost buffle plate 11 in order to prevent a low-temperature corrosion by sulfuric oxides contained in a combustion gas flowing through the heat transfer pipe 8.