To simplify structure and remove limitation in installation of downcast pipes by obliquely providing a corner heat receiving panel at a corner of a furnace in which the wall surface consist of furnace wall pipes and permitting the furnace wall pipes as the corner which is shut out by the corner heat receiving panel to communicate with liquid in a steam separating drum so as to be used as downcast pipes.

A corner heat receiving panel 15 is provided so as to bridge the corner made by a wall surface 2 and another adjacent wall surface 2, and a heat insulating material 18 is provided at the back surface of the panel, so that the portion of a triangle including the panel 15 as its leg is physically and thermally shut out from the inside of a furnace 1. Furnace wall pipes 16 of the panel 15, as well as furnace wall pipes 3 of the wall surface 2, communicate with a steam separating drum from the upper portion by way of an upper header, and the lower end communicates with downcast pipes 20 by way of a lower header. Thus the downcast pipes are not required to be separately provided, and hence only a small space is needed for the installation and it is not necessary to consider connection with other structure such as a steel frame and the like. As a result, the structure can be simplified, the thickness of the piping can be reduced, and also, the weight can be reduced.