This paper proposes a polarization-selective light emitter that can enhance preferentially the spontaneous emission rate of one desired polarization state using a one-dimensional metal grating mirror. Systematic numerical simulations were performed to determine the optimized structural parameters of the metal grating mirror consisting of ITO/silver, in which the two orthogonally polarized lights reflected from the grating mirror undergo completely opposite phases. This metal grating mirror was incorporated into a GaN medium, and the spontaneous emission rate of one linearly polarized light was 1.3 times higher than that of the other at a specific distance between the light source and mirror. In addition, the polarization ratio can be increased to 15:1 by considering the extracted power in a practical vertical GaN slab light-emitting diode structure. This study will be useful for demonstrating high-efficiency polarization-selective light-emitting diodes without using additional optical components, such as a polarizer.