A linearly polarized light beam is applied to the surface of a film (9b) and is reflected therefrom. The beam is then split into three light beams by three or four optical flats (13a - 13d). These light beams are applied to photoelectric conversion devices (18a - 18c) after passing through analyzers (14a - 14c) with fixed analyzing angles. The photoelectric conversion devices convert the beams into electric signals representing the intensities of these light beams. Two ellipsometric parameters ψ and Δ are calculated from these three electric signals.