There are several methods capable of determining he magnitude of optical anisotropy, such as spectrometric ellipsometry and polarized light reflectometry. The interference method is estimated to be no influence of surface scattering. The magnitude of anisotropy is a-As₄₀Ge₁₀S₃₅Se₁₅ thin film is analyzed by the reflection interference analysis method based on the difference depending on a phase of s- and p-polarized light. The theoretically analyzed value is compared with the result obtained by the measured technique.