An experimental study was conducted to investigate the polarization properties of the specularly reflected light from an anodic porous alumina (APA) membrane in which gold (Au) is embedded into the nanopores of APA by alternating current electrolysis. Consequently, we revealed that, when the Au-embedded APA membrane is considered to have the arrays of Au nanoparticles with rod-like shapes, the specularly reflected light changes rapidly from right- to left-circular polarization via linear polarization with respect to a slight variation in the angle of incidence of the incident light at a wavelength of 532 nm. The variation in the state of polarization, which is measured as a rapid change in the Stokes parameter s3, can be attributed to the simultaneous occurrence of the Fabry–Pérot interference in the single layer of APA and the excitation of localized surface plasmons on Au rod-like nanoparticle arrays. The rapid change in s3 has the potential to provide the effective information about the structural parameters of the Au-embedded APA membrane, such as the dimension of the Au rod-like nanoparticles or the refractive index of materials filling the nanopores of the APA layer.