Abstract: An ellipsometer has a nonpolarization beam splitter (18) for dividing reflected light (17) from an object to be measured (16) into portions traveling along first and second optical paths (18a, 18b), an analyzer (19) for passing the polarized light component in a reference direction of the reflected light portion traveling along the first optical path, and a polarization beam splitter (20) for dividing the reflected light portion traveling along the second optical path into two polarized light components in different directions with respect to the reference direction. The light beams passing through the analyzer (19) and polarization beam splitter (20) are sensed by first, second and third photodetectors (21a, 21b, 21c), respectively. In a coating thickness control method, first and second ellipsometers (35a, 35b) are placed before and after a coating apparatus (36) provided along the transport path of a belt-like plate to be coated (31). A first ellipsoparameter (.DELTA.1,.psi.