A plasma-assisted processing apparatus comprises: a vacuum vessel defining a processing chamber, a gas supply line for carrying gases into the processing chamber, a workpiece support for supporting a workpiece, disposed in the processing chamber and serving as an electrode, a disk antenna for radiating a high-frequency wave of a frequency in the VHF or the UHF band into the processing chamber, a high-frequency waveguide for guiding a high-frequency wave to the disk antenna, and a window of a dielectric material isolating the disk antenna from the processing chamber. A conductive ring is disposed between the disk antenna and the window such that its end surface is in contact with a peripheral part of the disk antenna.