Abstract: A permeable container having a plurality of holes whose diameters are equal to or less than two times the sheath length is provided in a vacuum container. provided at the bottom of this permeable container is an electrode on which an object to be processed is to be placed. A high-frequency power supply is connected to this electrode, and the permeable container is grounded. The permeable container and the electrode are insulated from each other by an insulator. When a process gas supplied into the vacuum container is guided into the permeable container through its holes and a high-frequency voltage is applied to the electrode, plasma is produced in the permeable container. Because the diameters of the holes are equal to or less than two times the sheath length, plasma is trapped inside the permeable container, thus improving the stability and density of plasma.