The fluorescence from a fluorophore on a multilayered substrate fabricated by a metal and a dielectric is known to be enhanced by more than 100-fold. In the course of this study, we prepared a multilayered substrate with Ag as the metal and ${\mathrm{Al}}_2{\mathrm{O}}_3$ as the dielectric and then investigated the effects of the polarization of the excitation light on the enhancement of the multilayered substrate. It was found that the enhancement was attributed to an electric field oscillating parallel to the substrate. Maximum 200-fold enhancement could be achieved with $80\mathrm{nm}$ thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ when an unpolarized excitation light was used with an incident angle of $20°$ .