PURPOSE: To perform monitoring and control readily and effectively, by monitoring the change in a light emitting state of gas plasma in etching operation all the time, and detecting abnormality based on the change in said light emitting state.

CONSTITUTION: A detecting sensor 21, which is located at the outside of a chamber 1, always monitors the change in the state of plasma, which is generated in an etching chamber 1a. The light signal, which is detected with the sensor 21, is taken out as a voltage output through, e.g., an optical transistor 23, a low-pass filter 24 and a voltage transformation circuit 25. The detected voltage signal 26, which is obtained in this way, and a preset reference voltage signal 28, which is obtained from a reference voltage generating circuit 27, are compared in a comparator circuit 29. Then the abnormality of the parameter of each operating condition and the abnormality of the equipment at the time of etching are quickly detected readily based on the change in plasma state. Thus the equipment is effectively monitored and controlled.