To reduce a spectral action generated on light emission without using a diffusion sheet, to improve color reproducibility, and to make a light guide plate using a diffraction grating high in luminance and thin in thickness originally in a backlight unit for a liquid crystal display device using the light guide plate with the diffraction grating disposed thereon.

A plurality of cylindrical lenses 7, or a plurality of prisms 10 made of a material with Abbe constant of ≤30 are disposed between the light guide plate 2 and a liquid crystal panel in parallel to distribution of the diffraction grating. Also, a reflection plate, disposed on the backside of the light guide plate, has a plurality of cylindrical lenses 11, or a plurality of prisms 15 made of a material with the Abbe constant of ≤30 in parallel to the distribution of the diffraction grating 4 on the light guide plate side surface thereof, and a light reflection layer 13 on the backside of the light guide plate.