PURPOSE: To reduce a change in luminous strength, by placing an earthing conductor which is caused to branch between a matching circuit and a low-frequency filter circuit of a line for connecting one electrode and a high-frequency power source.

CONSTITUTION: A first electrode 1 and a second electrode 2 are positioned facing and opposite to each other at the inside of a high-frequency lamp 5 mounted between magnets 7 and 8. The electrode 2 is earthed outside the lamp 5, and is connected to a high-frequency power source 12. Meanwhile, the electrode 1 is connected to a power source 12 outside the lamp 5 via a matching circuit 9 and a low-frequency filter circuit 11, and an earthing line is caused to branch between the circuits 9 and 11. A high-frequency filter circuit 10 is situated in the middle of the earthing line. This enables the DC potential of the electrode 1 to produce an earthing potential like that of the electrode 2 does. However, the presence of the circuit 10 prevents the high-frequency potential of the electrode 1 from being affected by the influence of the earthing. This keeps a DC potential of a high-frequency plasma in a constant state, which results in stabilizing luminescence from the plasma.