Abstract: A plasma processing apparatus includes: a chamber; a substrate support provided in the chamber; a bias power supply that supplies an electrical bias energy to an electrode of the substrate support; a matching box including a matching circuit; a radio-frequency power supply that supplies a radio-frequency power having a variable frequency into the chamber through the matching box, and adjusts the frequency of the radio-frequency power in each of a plurality of phase periods within the cycle of the electrical bias energy; a sensor that detects an electrical signal reflecting a deviation of a load impedance of the radio-frequency power supply from a matching state; and a filter that generates a filtered signal by removing and an intermodulation distortion component of the radio-frequency power and the electrical bias energy from the electrical signal in each of the plurality of phase periods.