Abstract: A plasma processing system controls the electronegativity of a plasma produced by ionizing a process gas when processing a substrate by using the plasma. The relation between the pressure in a processing vessel (1) and the frequency of a RF power source (11′), and the electronegativity of the plasma produced by the agency of RF power is determined beforehand. A controller (18) adjusts the pressure in the processing vessel (1) and/or the frequency of the RF power source (11′) in a real-time control mode by a feedback control operation on the basis of a pressure measured by a pressure sensor (17) and a frequency measured by a frequency meter (15) to adjust the electronegativity of the plasma to an appropriate value. The electronegativity of the plasma can be determined through simulation using a one-dimensional RCT model of the plasma.