To solve problems such as softening of weld metal or a heat affected part, marked deterioration of toughness, corrosion resistance, etc., and resultant lowering of joint efficiency of a weld joint to 50-80%, which are caused by change in materials through high temperature heating in a weld zone of various metallic materials and metallic clad materials at the time of welding, and also problems such as generation of fatigue fracture and stress corrosion crack in the toe during the use which are due to reinforcement of weld on the surface and generation of residual stress.

In this method, a weld zone and a casting material or the like are processed by peening on the material surface for minute deformation using an impulse wave by generating a high pressure plasma with laser of a short pulse and a high peak output. Thus, by obtaining a compressive residual stress, the strength of a weld joint or a casting material is enhanced, so that fatigue strength and corrosion resistance or the like are improved.