PURPOSE: To provide the weld zone of full penetration without generating the sag of the molten pool and the projecting bead mainly at the part to be welded where the welding position is successively changed.

CONSTITUTION: In a welding method where the plasma is the heat source, the capacity of the plasma arc is periodically changed by periodically changing the flow rate of the plasma gas. It is preferable that the period to change the flow rate of the plasma gas is relatively low below 10Hz. Concerning the flow rate of the plasma gas, it is preferable that the peak flow rate is ≥1 litre/min. in the case of the plasma key hole welding, while the peak flow rate is ≤3 litre/min. in the case of the non-key hole welding. As for the mechanism to fluctuate the flow rate of the plasma gas, e.g. a needle valve for setting the base gas, a needle valve for setting the peak gas, and an ON-OFF solenoid valve are provided in parallel between a gas adjuster and a plasma arc welding machine, or between the gas adjuster and the welding torch. This welding method is mainly suitable for the case of the plasma arc welding of the part to be welded where the welding position is successively changed.