A scaffold assembling of a large component for a boiler requiring inspection is not performed by a high-place work but assembled safely and in a short time. The scaffold is assembled to the large component for the boiler supported by a component supporting beam at the ground side before it is attached to a steel frame main-beam 16. Scaffold lifting/supporting beams interposing coil spaces of a superheater 14 are suspended on wires 36 lifted down from spaces of a ceiling tube wall 34 being a horizontal member of the superheater 14. The scaffold lifting/supporting beams 32 are lifted to raise little by lithe within the superheater, and scaffold components are assembled sequentially toward downward. After a required number of steps of scaffolds are assembled, the scaffold lifting/supporting beams 32 are coupled to the ceiling tube wall 34 to be supported. Finally, the superheater 14 is raised by a lifting means to install to the main-beam.