PURPOSE: To make it possible to easily repair a damaged part by use of welding, by heating an outer surface cover along two ring-like lines close to the damaged part on both side thereof and along the outer periphery of the cover so that the self-fastening effect of the outer surface cover is partly released.

CONSTITUTION: Two ring-like heating lines 5 along the outer periphery of an outer cover 2 and close to a damaged part 4 to be repaired on axially both sides of the latter are externally heated by a band heater to allow the outer surface cover 1 to thermally expand in part so that the self-fastening action of that heated parts is partly released. The outer surface cover 2 expands thermally so that a gap is formed between itself and an internal member 3, and in such a condition the damaged part 4 is repaired by welding. After repairing, the outer surface cover 2 is cooled, naturally. Then, the outer surface cover 2 is returned again into its self-fastening condition, but since it is made of a thin sheet, strain and residual stress in the repaired part are relaxes at most. Accordingly, it is possible to eliminate extra strain and deformation.