PURPOSE: To equalize inner fluid fed from the lower part of a furnace in an upper part and to minimize temperature difference of the part to be overheated in a dynamic state by allowing adjacent vaporizing tubes for forming a furnace wall to communicate with one another at an intermediate part between an upper header and lower header.

CONSTITUTION: A plurality of furnace wall tubes 1 for forming a furnace wall are externally bent along upper and lower horizontal surfaces in an intermediate part A between upper headers 4, 5, 6, 6 and lower header 2, 3, 3, and connected to and intermediate header 7 disposed out of the wall surface. Accordingly, since the adjacent tubes are connected through the intermediate header, even if dryness becomes irregular in the lower part of the furnace, irregular fluid is mixed to be made uniform in the upper part of the furnace to minimize fluid temperature unbalance generated in an outlet of the furnace. Therefore, there is no fear of generating an unreasonable stress at the wall.