We experimentally investigate the polarization property of the specularly reflected light from an anodic porous alumina (APA) membrane coated with a gold (Au) thin film. As a result, we reveal a rapid change in the normalized Stokes parameter s 3 of the specularly reflected light around the angle of incidence θ A at which the resonance absorption of the incident light occurs. The rapid change in s 3 demonstrates that the specularly reflected light can rapidly be right- to left-elliptically polarized via linear polarization at the zero-crossing point θ Z of s 3 . Moreover, θ Z is located close to θ A , and θ Z as well as θ A depend on the occurrence conditions of the resonance absorption. From numerical aspects based on the Maxwell Garnett effective medium approximation, we clarify that the rapid change in s 3 accompanies the Fabry–Pérot (FP) resonance in the Au-coated APA membrane. The numerical results also suggest that the change in the refractive index of the filling material into nanopores of the Au-coated APA membrane can be successfully estimated by using the rapid change in s 3 .