A plasma processing apparatus includes a plurality of radio-frequency power supplies for supplying radio-frequency powers having frequencies different from each other, a common feeding line for superposing radio-frequency powers supplied respectively from the plurality of radio-frequency power supplies and feeding the superposed radio-frequency power to a same radio-frequency electrode, a radio-frequency power extracting device for extracting radio-frequency powers having predetermined frequencies from radio-frequency powers fed via the feeding line, and a radio-frequency voltage detector for measuring voltages of the radio-frequency powers having the predetermined frequencies extracted by the radio-frequency power extracting device.