PURPOSE: To obtain better processability for the change of an electric current value within a wide range, by using a field-effect transistor as a switching element of an inverter.

CONSTITUTION: The output of a direct current source 10 is converted into a high frequency output by the inverter 28, which is then applied through current restricting resistances 20 and relay contacts 22 to a discharge electrode 16 to perform the processing. A field-effect transistor 30 is used as a switching element of the inverter 28. Accordingly, irrespective of the magnitude of the switching current value, switching operation at a high speed can be obtained, and a pulse electric current wave shape having sharp building up and trailing from the inverter can be obtained.