To improve the visible transmittance of switchable mirrors based on Mg–Y alloys in the transparent state using antireflection layers, optical indices n and κ of the hydrogenated switchable mirrors were evaluated using spectroscopic ellipsometry in the range of 310 to 1650nm. The Y composition z in the Mg1−zYz alloys was varied between 0.51 and 0.7. The evaluated spectra both n and κ in the visible range increased with decreasing wavelength and increasing Y composition. Furthermore, the wavelength where the k value increased sharply was increased with Y composition. As a result of estimation of the effect of antireflection (AR) coating, it is expected that a switchable mirror using Mg0.39Y0.61 covered with AR layer with the refractive index of ~2.3 and ~50nm thickness improved to be Tvis of over 60%, while the switchable mirror without AR had Tvis of 34%. Thus, it is concluded that AR layer is effective to improve Tvis in the transparent state.