To measure a finely processed pattern on real-time basis on the spot and to calculate a state that an element is finely processed from the measured value to aim to form a desired shape to be processed of the element.

The initial processing conditions for an element are set to start the etching work for the element and after an elapse of a prescribed time, a luminous flux from a laser beam source 24 is turned into a linearly polarized light by a polarizer 25 to irradiate the linearly polarized light on a substrate S, elliptically polarized light reflected from the substrate S is detected by a detector 28 through a compensator 26 and an analyzer 27, and the polarization state at the time is detected from the azimuthal angle of the rotation of the detector 28. Moreover, the processing conditions are changed so that the processed state of the element at this time enters within the range of the set point to form the desired element.