PURPOSE: To detect the end point of etching accurately even if the wafer is etching continuously by applying auto gain and offset so that the voltage of a wafer to be treated on overetching is equal to that of the wafer which is treated immediately before.

CONSTITUTION: A first wafer is etched and the luminous level voltage halfway during etching and that on overetching are calculated by an operation means 21. Then, a second wafer is etched, the luminous level voltage is calculated by the operation means 21, the gain offset and the offset compensation value of two wafers are calculated based on the operation value and the operation value data of the first wafer output from a storage means 22, and then the luminous signal operation value of the second wafer is compensated based on the compensation values. The compensation is made while the value halfway during etching and the value on overetching are equal to those of the first wafer. Therefore, by detecting the end-point voltage value obtained from the first wafer, the end point of the etching can be detected.