Plasma processing of plural substrates is performed in a plasma processing apparatus, which is provided with a plasma processing chamber having an antenna electrode and a lower electrode for placing and retaining the plural substrates in turn within the plasma processing chamber, a gas feeder for feeding processing gas into the processing chamber, a vacuum pump for discharging gas from the processing chamber via a vacuum valve, and a solenoid coil for forming a magnetic field within the processing chamber. At least one of the plural substrates is placed on the lower electrode, and the processing gas is fed into the processing chamber. RF power is fed to the antenna electrode via a matching network to produce a plasma within the processing chamber in which a magnetic field has been formed by the solenoid coil. This placing of at least one substrate and this feeding of the processing gas are then repeated until the plasma processing of all of the plural substrates is completed. An end of seasoning is determined when a parameter including an internal pressure of the processing chamber has become stable to a steady value with plasma processing time.