PURPOSE: To make the most of plasma welding so as to stably execute formation of key hole and deep penetration welding without generating defect of burn through, etc., of molten pool.

CONSTITUTION: The gas plasma welding method is that a gas flow rate of plasma gas flow is periodically varied in pulse state. Plasma gas flow is varied so that a period of peak gas flow rate Q1 and a period of base gas flow Q2 are alternately varied in pulse state and the period of base gas flow Q2 is 0.3 to 1 times of the peak flow rate Q1, the base flow rate Q2 is 0.3 to 0.7 times of the peak gas flow rate Q1. Further, between the peak gas flow period and base gas flow period, there is a period of one or ≥2 of the flow between the peak gas flow rate and base gas flow rate. Further, a filler metal wire is supplied into the molten pool formed backward in the welding advancing direction by plasma arc column, the method is suitable for welding a horizontal fixed tube in all positions.