A coaxial passage for supplying microwaves is vertically provided over a dielectric window forming part of a processing chamber, and a conductor plate for transmitting the microwaves is arranged over the dielectric window which is positioned on the outer circumference of the coaxial passage. A microwave transmission passage can be formed between the surface of a plasma so produced as to have a critical electron density n.sub.c, and the conductor plate thereby to transmit microwaves without any reflection along the dielectric window uniformly from the center to the outer circumference.