A ring mechanism, comprising a focus ring and divided cover rings surrounding a wafer W placed on a loading table (lower electrode) mounted in a processing chamber, wherein a ring-shaped clearance δ1 is provided between the divided rings to spread plasma to the radial outside of the focus ring to allow plasma to get therein, whereby a potential difference between the wafer W and the focus ring can be eliminated to prevent arc discharge by plasma from occurring between the wafer W and the focus ring.