To prevent occurrence of arcing caused by difference of thermal expansion between the electrode and the solid dielectric in a plasma processing apparatus.The bottom part of a casing 20 of processing units 10L, 10R is open, this opening part is closed with a solid dielectric plate 50, and an electrode 30 is received in the casing 20 such that the electrode 30 is free in the longitudinal direction. The solid dielectric plate 50 has such strength as capable of supporting the dead weight of the electrode 30 solely by itself. The electrode 30 is placed on the upper surface of the solid dielectric plate 50 is a non-fixed state such that the dead weight of the electrode 30 is almost totally applied to the solid dielectric plate 50.