Cores are embedded in a cladding, each core constituting a continuous optical path in which a first core end surface is exposed at a first end surface, and a second core end surface is exposed at a second end surface. The plasma processor is provided with an electric potential control DC power supply 33 to control the electric potential of this focus ring 9, and so constituted that the lower electrode 4 is supplied with a DC voltage of e. The central portion is preferably curved, with a concave side of the curvature facing the rivet formation.