To satisfactory determine the etching end point by the mode decomposition of an input pattern, using an orthogonal function system, in which the time change of the spectrum intensity of a plasma light is set, and determining the etching end point, based on the coefficient values of specified decomposed mode.

An extracted plasma light is converted into a voltage signal through a monochromator and a photomultipier tube and sent to an input part 21 via an A/D converter, an output signal from the input part 21 is sent to a pattern decomposing step 24 via a noise filter 22 and signal pattern forming step 23, a sent pattern function is decomposed for each degree component, using orthonormal functions, only specified determining elements are sent to an end point determining logic 25 which sends the determined result to an output part 26, and the output part 26 gives a detection signal to an etcher controller when the determined result indicates the etching end point, whereby even when the exposed area ratio is low, the etching end point can be satisfactorily determined.