A plasma processing system includes a process chamber equipped with a gas supply unit, a gas exhaust and an electromagnetic energy supply unit for generating plasma from process gasses, thereby subjecting a specimen placed on a specimen stage to a plasma process. The system includes a spectrometer detecting a spectrum of plasma emission generated in the chamber, flow controllers controlling flow rates of process gasses to be supplied, and a controller controlling the flow controllers. The controller includes a calculation unit for calculating an amount of reaction byproducts generated in the chamber, in accordance with the spectrum of the plasma emission detected with the spectrometer and an input unit for inputting a target timeline of the amount of reaction byproducts, and controls amounts of the process gasses such that a calculation result of the amount of reaction byproducts becomes coincident with the input target timeline.