PURPOSE: To obtain a high output by small suppressing a current value so as to prevent consumption of a tungsten electrode by arranging a plurality of plasma operating gas supply flow paths in the periphery of the electrode, and decreasing a flow speed of plasma operating gas lower in accordance with reaching the periphery.

CONSTITUTION: Operating gas from an operating gas supply flow path 6 is divided into two of the first/second operating gas supply flow paths 4, 5 by a speed adjuster 7, further adjusted into respective supply speeds and supplied from the first/second operating gas supply flow paths 4, 5. When a relative speed to the atmosphere is decreased by decreasing a supply speed of operating gas 5 in an outer side, turbulent energy is small suppressed to generate a more stable flow of operating gas 4 in an inner side. In this way, high output operation of small suppressing a current value can be performed by decreasing a roll-in of the atmosphere, and also consumption of an electrode by Joule heat can be suppressed to a minimum limit.