PURPOSE: To stabilize plasma and to prevent a nozzle from being damaged by setting the orifice diameter of a nozzle of a plasma torch and electric current density of the orifice cross-sectional area in the specific range respectively.

CONSTITUTION: The diameter (d) of an orifice 2b of the plasma torch 1 is regulated to 1-10mm and the current density of the cross-sectional area in the orifice 2b is set to 2-5A/mm2 to perform plasma machining. When mixed gas of monoatomic gas with polyatomic gas is used as plasma gas to perform machining by the plasma torch 1, the current density is always above the unstable region of the plasma and further, exists in the range where the nozzle 2 can be prevented from being damaged. The current density range is regulated by regulating the orifice diameter (d) to 1-10mm. By this method, the plasma is stabilized and the nozzle 2 is prevented from being damaged.