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./n therebetween, is electrically connected with the plurality of split coils of the toroidal coil so that two magnetic fields generated by the each pair of split coils opposing to each other are directed in the same direction in parallel to each other in the processing chamber, and the plurality of currents are respectively flowed into the respective pairs of split coils opposing to each other, thereby generating a rotating magnetic field in parallel to surfaces of the plane-parallel electrodes.