To accurately detect an etching end point.

Using an optical or electric signal from a plasma, a frequency plasma variation (step 220) during a wafer manufacturing process is monitored (step 210). For detecting an end point, (for example) the amplitude change of a plasma glow at a specified audio frequency selected so as to obtain a sensitivity with respect to a material to be etched is used to generate an end point signal (step 230). This signal has a potential response time equal to a min. filtering time by one period of the selected frequency plus noise reduction. To extract effective parameters from the plasma glow, a frequency analyzing DSP or simple frequency filtering method is usable.