In a high-frequency power source, a malfunction is prevented by precisely removing harmonic components or a modulated wave component which develops while producing a plasma, and a proper high frequency power can be impressed on a plasma processing apparatus. The high-frequency power source includes a power monitor constituted of a directional coupler, a mixer, a 100 kHz low-pass filter, a low-frequency detector, and an oscillator. A 100 MHz high-frequency wave including modulated wave components and the like extracted by the directional coupler and 99.9 MHz high-frequency wave oscillated by the oscillator are added by the mixer. An output of the addition is converted by the low-frequency detector into 100 kHz, resulting in detection.