To suppress occurrence of an excess bending stress at starting or stopping at a welded part of a boiler furnace having a spiral wall and to enhance reliability.

In the boiler furnace comprising a spiral wall 5 in a furnace wall 1 and supported to supporting reinforcing bars 6, twist preventive fitments 4 for preventing twisting of the overall furnace due to a temperature deviation are provided at least two positions of the wall or preferably four corners of a lower part of the wall 1, and the wall 1 is coupled to the bars 6 through the fitments 4. When such a twist preventive structure is adopted, twisting deformation of the overall furnace apprehended at the time of starting and stopping can be prevented by the fitments 4. Accordingly, an excess bending stress is not generated at the welded part of the wall 1 to a ceiling wall 3 to suppress a fatigue damage, and hence a crack is scarcely generated at the welded part or a heat transfer tube to improve the reliability.