A linearly polarized light beam is applied to the surface of a film and is reflected therefrom. The beam is then split into three light beams by three or four optical flats. These light beams are applied to photoelectric conversion devices after passing through analyzers with fixed analyzing angles. The photoelectric conversion devices convert the beams into electric signals representing the intensities of these light beams. Two ellipsometric parameters .psi. and .DELTA. are calculated from these three electric signals.