To provide a structure for hang change of the upper part of a supporting tube of a boiler header, in which a fiducial point for movement of a header supporting tube to the furnace wall of a boiler main body is defined, and a difference in movement between them is minimized, a constant hanger of a great-capacity and a support-tube stopper are eliminated, and structure simplification and cost-reduction can be attained.

The level of the upper hanging-change point P2 of a header- supporting tube 25 is allowed to agree substantially with the level of a hanging point P1 of the boiler main body 1.