PURPOSE: To easily and smoothly clean a gaseous reactant in a plasma working device using the gaseous reactant.

CONSTITUTION: A body 2 to be worked is inserted into a working vessel 1, an electric discharge is generated between the body and an electrode 3 by a discharge power source 9 to produce plasma, a gaseous reactant is supplied to the plasma from a feed port 1a, and the body is worked. An electromagnetic wave cleaner 12 is furnished to an exhaust port 1c for the gaseous reactant to clean the reactant. A nozzle having a tapered reflecting surface is irradiated with an electromagnetic wave by a lens so that reflection is repeated on the tapered surface, and the exhaust gaseous reactant is introduced into the nozzle from a feed port communicating with an exhaust port 1c, circulated and passed to clean the reactant.