PURPOSE: To plan the improvement of workability by removing a holding member after partially cutting a safe and inner than the base welding part of a thermal sleeve to insert the holding member and exchanging a duplex tube part outer than the cutting part for a new one.

CONSTITUTION: Water level is decreased down to the repair water level to make a cylindrical hollow part 13 around a thermal sleeve 9 gas atmosphere and a safe end is approximately half cut to drill a hole 14. Next, after a holding member 15 is inserted in a hollow member 13 to fix the sleeve 9, a piece of outer tubing is cut from the neighborhood of the base welding part of a nozzle 4 and the end 7. Next, the part between the inner end part of a repair safe end in which a cutout part 16a previously corresponding to a cutting part is formed and the edge of the nozzle 4 is welded 18a. After the member 15 is cooled and transformed to remove from the hollow part 13, a blocking piece is engaged with the cutout part 16a and welded to restore.