[en] Stresses greater than the yield point of materials of structures and equipments in the inside of a reactor pressure vessel undergoing neutron irradiation are applied to positions near positions to be welded. For example, water stream jetted out of a nozzle at a high speed is collided against the surface of the structures, so that cavitations are formed by water jetting to apply plastic fabrication on the surface of the structures thereby increasing dislocation. Heat treatment of keeping at a temperature range of from 500 to 900degC for 1 to 60min., and then rapidly cooling is applied to gather He present in the inside of the materials onto the dislocation thereby forming bubbles. Subsequently, welding is performed for repair. Since the amount of He moved to a grain boundary by the heat of welding is reduced, formation of bubbles of He at the grain boundary can be suppressed to prevent occurrence of welding cracks due to lowering of strength at the grain boundary. Accordingly, repair of crack-like flaws such as stress corrosion crackings can be repaired by welding. (I.N.)