Abstract: A flange, which forms a portion of a vacuum container, has a rectangular opening surrounded by an insulating frame. A plate-shaped radio-frequency antenna conductor 13 is provided so as to cover the opening, with the insulating frame clamped thereby. In this structure, a radio-frequency power source is connected via a matching box to one end along the length of the radio-frequency antenna conductor, the other end is connected to ground, and electric power is supplied so that a radio-frequency current flows from one end of the radio-frequency antenna conductor to the other. By this method, the impedance of the radio-frequency antenna conductor can be lowered, and high-density plasma with a low electron temperature can be efficiently generated.