To non-destructively measure the carrier concn. and thickness of a semiconductor layer at once by improving the ellipsometric method.

An optical source 4 of an ellipsometric apparatus emits a light (e.g. infrared) at such a frequency band that the optical absorption by free carriers in a semiconductor occurs and the light passed through a polarizer 5 is irradiated on a carrier-doped semiconductor layer 9. Reflected light is received by a photo detector 6 through a polarizer 7. Using the apparatus 1, the reflection amplitude ratio tan Ψ and its phase Δ are obtd. Assuming the carrier concn. Nc and film thickness d, Ψ and Δ are computed according to specified equations and the curve fitting is made so as to reduce the errors of measured values of Ψ and Δ from computed values and when the errors are within allowed ranges, the assumed values of Nc and d are taken as measured values of the semiconductor carrier concn. Nc and thickness d.