PURPOSE: To provide an RF power source for a vacuum tube type plasma device of long life.

CONSTITUTION: After high frequency oscillated from an RF oscillator 1 is amplified by a first power amplifier 2, the high frequency is amplified by a second amplifier 3, and is fed from a terminal To to a plasma device 6. A signal corresponding to the plate current of a vacuum tube in the second amplifier 3 is fed to a limiter circuit 7, in which a signal is fed to a filament voltage control circuit 8 when the plate current becomes less than a preliminarily set threshold. A signal corresponding to the output of the first amplifier 2 is fed from a power detector 4 to the filament voltage control circuit 8. When a signal indicating that the plate current is lower than the threshold, is fed from the limiter circuit 7, the filament voltage is increased according to the monitor signal from the power detector 4, by the control circuit 8.