PURPOSE: To reduce the cost of manufacturing the whole of a boiler by reducing the required numbers of soot blowers and of intermediate support pipes and increase a support span without use of such an intermediate support pipe.

CONSTITUTION: A horizontal heat transfer pipe 10 of a boiler is constructed such that it comprises a plurality of horizontal pipe portions 12 and a plurality of U-shaped pipe portions 14 for alternately coupling adjacent end portions 12 of the horizontal pipe portions and it constitutes a hollow pipe communicated in a vertical plane as a whole. The horizontal pipe portion includes a fin extending vertically, and at least parts of the fins adjoining vertically are fixed to each other through welding.