PURPOSE: To realize a monitor with a simple constitution and, further, judge the reproducibility of a plasma density and determine the limit of the usage of etching equipment by a method wherein the light path length difference of laser beams caused by the variation of the plasma density is monitored as a voltage by using specific detection devices.

CONSTITUTION: The status of a plasma is monitored by employing:a laser 1; a quarter wavelength plate 2 which converts a linear polarization beam from the laser 1 into a circular polarization beam; a polarization prism 3 which divides the circular polarization beam into two laser beams whose polarization planes are perpendicular to each other; a lens 4 which makes the two laser beams, i.e. a reference beam 5 and a test beam 6, parallel to each other; a compensator 18 which can control a Gradstone-Deret constant which varies in accordance with medium gas; a lens 7 and a polarization prism 8 which recombine the two laser beams 5 and 6 into one; a compensator 9 which can control the phase relation between two laser beams whose polarization directions are perpendicular to each other; a polarization prism 10 which divides one laser beam into two interfered beams; and detectors 11 and 12 which convert the intensities of the interfered beams into voltages.