PURPOSE: To ignite a plasma without being restricted by position relation of an electrode and a nozzle by attaching a conductive material which deforms based on temperature alteration to either a nozzle or an electrode.

CONSTITUTION: Before plasma ignition, the gap between an electrode 1 and a material 12 which deforms based on temperature alteration is so set to have a distance as to cause dielectric breakdown by high frequency voltage. After high frequency voltage is applied and plasma is ignited, the conductive material 12 receives electric heat form the plasma and deforms and escapes to a recessed hollow place in the inner circumference of the nozzle 3. As a result, the diamaters of the nozzle and the electrode can be selected, without being restricted by the position relation of the electrode and the nozzle.