The present invention aims at providing a radio-frequency antenna unit capable of generating a high-density discharge plasma in a vacuum chamber. The radio-frequency antenna unit according to the present invention includes: a radio-frequency antenna through which a radio-frequency electric current can flow; a protective tube made of an insulator provided around the portion of the radio-frequency antenna that is in the vacuum chamber; and a buffer area provided between the radio-frequency antenna and the protective tube. The “buffer area” refers to an area where an acceleration of electrons is suppressed, and it can be formed, for example, with a vacuum or an insulator. Such a configuration can suppress an occurrence of an electric discharge between the antenna and the protective tube, enabling the generation of a high-density discharge plasma in the vacuum chamber.