PURPOSE: To perform etching under the optimum conditions by keeping a fixed distance between plasma and a semiconductor wafer.

CONSTITUTION: High-frequency waves are applied on parallel flat electrodes 4 from a high-frequency generating means 5, and a plasma discharge area is detected by a detecting means 7. When the detection signal is too far from the prescribed position of a semiconductor wafer 6, the high-frequency power of the high-frequency generating means 5 is increased and when the detection signal is closer than the prescribed position of the semiconductor wafer, the high-frequency power of the high-frequency generating means 5 is reduced. Thus, a control means 8 performs automatic control and a distance between the plasma area and the semiconductor wafer is always kept fixed. The constitutive component in a vacuum process chamber 1 and a chamber 1a which is a container for shielding the inside of the chamber from the outside air and keeping the atmosphere are all composed of quartz.