Method of subjecting welded structure to heat treatment

A method of subjecting a welded structure to heat treatment in which a tensile yield is produced on an inner surface of the welded structure and a compressive yield is produced on an outer surface of the welded structure by the steps of placing a cooling medium in the interior of the welded structure and heating the outer surface of the welded structure to cause a difference in temperature to exist betweem the inner surface and the outer surface of the welded structure, the method comprising the steps of locally heating a weld by a plurality of heating members arranged along the weld to improve residual stress in a localized area of the weld, and repeating the improvement of residual stress in another localized area of the weld so that residual stress in the whole of the weld is improved.