Abstract: In a plasma processing s including a processing chamber and plate-parallel electrodes, provided in the processing chamber, for generating a high-frequency electric field in response to a high-frequency voltage, a ring-shaped core is provided in the periphery of the processing chamber, and a toroidal coil includes a plurality of 2n split coils wound in all the periphery of the ring-shaped core so that each pair of split coils oppose to each other. An alternating-current power source for generating a plurality of n-phase alternating-current currents including a plurality of n currents having a phase difference of either .pi./n or 2.pi.