PURPOSE: To provide a plasma arc welding method capable of making plasma arc passing through a material to be welded so as to form stable penetration bead in the case of a thick material to be welded.

CONSTITUTION: A root gap of materials 1 to be welded is made to ≥2mm, a backing strip is arranged on the rear side of groove. The mixed gas consisting of ≥2vol% helium and the balance argon gas as an plasma gas is supplied at a flow rate of ≥4l/min. Further, in the case the plasma gas flow rate is fluctuated in pulse, its peak flow rate is ≥4l/min.