A plasma processing method for a plasma processing apparatus which includes, a gas ring, a bell jar, an antenna, a sample table, a Faraday shield, and an RF power source circuit for supplying a power source voltage to the antenna and the Faraday shield. The RF power source circuit includes an RF power source, an antenna connected with the RF power source, a resonance circuit connected in series with the antenna and supplying a resonance voltage, a detection circuit for detecting the resonance voltage of the resonance circuit, and a comparator circuit for comparing the resonance voltage detected by the detection circuit with a predetermined set value. A RF bias voltage is adjusted based on the result of comparison by the comparison circuit.