The invention relates to a backlight unit for liquid crystal displays, etc.; and its object is to provide a backlight unit not involving the problem that the emitted light leaks out of the optical waveguide, even when the space around the cold-cathode tubes in the light source unit for it is filled with a liquid of which the refractive index is nearly the same as that of the glass material that forms the outer wall of the cold-cathode tubes. The backlight unit comprises a housing 6 which houses cold-cathode tubes 2, 4 therein and of which the inner surface is coated with a light reflector 10; a transparent liquid filled in the housing 6; and an optical waveguide 1 made of a transparent substance and having a light-emitting surface S. The reflective surface of the light reflector 10 has a cross-section profile of X-T-U-V-W-Y, on which the light emitted by the cold-cathode tubes 2, 4 is reflected, and the thus-reflected light is led to the light-emitting surface S of the optical waveguide 1 at an incident angle not smaller than the critical angle to the surface S.