1. A method for measuring at least one of a refractive index, an absorption coefficient and a thickness of one layer of a thin film formed on a substrate, in which the thin film has m (m.gtoreq.1) layers comprising a transparent uppermost layer which is designated a first layer, in which a measured sample having the thin film and the substrate includes a total of (3m+1) parameters comprising a refractive index n(0) of an incident medium, a refractive index n(j) (j=1 to m) of a j-th layer, absorption coefficients k(j) (j=2 to m) of second to m-th layers, a refractive index n(m+1) of the substrate, an absorption coefficient k(m+1) of the substrate, and thickness d(j) (j=2 to m) of the second to m-th layers, and in which (3m) of the (3m+1) parameters are known and one of the (3m+1) parameters is unknown, said method comprising the steps of: selectively irradiating respective monochromatic beams of s-polarized light and p-polarized light on said thin film at a predetermined incident angle; receiving said s-polarized light and said p-polarized light after they are reflected from said thin film and determining a reflectance Rs of said s-polarized light and a reflectance Rp of said p-polarized light on the basis of said received s-polarized and p-polarized lights; determining a function of F(x) including unknown parameter X as a variable by using respective values of said (3m) known parameters, a value of said incident angle .theta.(0), a value of wavelength .lambda. of said monochromatic beam, a value of said reflectance Rs and a value of said reflectance Rp; numerically solving an equation of F(x)=0 to thereby obtain a value of the unknown parameter X, and outputting a signal representing said obtained value as the value of said unknown parameter.