PURPOSE: To prevent the generation of a nitride layer at the cut end face and to decrease the blow hole in the stage of welding said part by using a gaseous mixture formed by mixing oxygen at a specific ratio or above with air as gaseous plasma and performing plasma cutting.

CONSTITUTION: A gaseous mixture formed by mixing air and oxygen and regulating the mixing ratio of the oxygen with respect to the air at ≥75% is used as gaseous plasma and plasma cutting is performed. Then, the amt. of the active gaseous nitrogen in the plasma jet is decreased and the generation of the nitride layer at the cut end face is suppressed. Not only the stage for removing the oxide layer prior to welding is omitted but also the defectless weld zone having no blow hole is obtd.