In a plasma processing apparatus, electrical bias energy is provided from a bias power supply to a substrate support. Source radio-frequency power is provided from a radio-frequency power supply to a radio-frequency electrode through a feed line. A phase period having a minimum value of a power level of a reflected wave of the source radio-frequency power is identified from a plurality of phase periods in a bias cycle of the electrical bias energy. A reference value being a phase difference between a voltage and a current on the feed line in the identified phase period is determined. A source frequency of the source radio-frequency power is controlled for each phase period based on a result of comparison between the reference value and the phase difference between the voltage and the current on the feed line in a corresponding phase period of the plurality of phase periods.