Disclosed herein is a backlight unit which has high utilization efficiency over the entire visible wavelength band. The backlight unit includes a reflective polarization splitter to allow a light of a surface light source to emit therethrough. The reflective polarization splitter includes a plurality of wires arranged in a wire grid shape. With this structure, when light of the surface light source passes through the wires, the wires passes only a certain polarized light, for example, s-polarized light having a vibration plane vertical to a longitudinal direction of the lattice wires therethrough, while reflecting other polarized components of the light according to the reflection coefficient of the wires.