A back light unit capable of achieving a reduction in thickness and manufacturing costs is disclosed. The back light unit includes light sources arranged in parallel, to emit light, a case receiving the light sources, a diffuser plate diffusing the light from the light sources, condensers laminated on the diffuser plate and having predetermined thicknesses, and a diffuser interposed between the adjacent condensers. The distance from the lamps to the diffuser plate is reduced by the diffuser plate and the optical sheets (three or four sheets) laminated over the diffuser plate, to reduce the total thickness of the back light unit. Also, the number of lamps is reduced by 2 to 4, as compared to the conventional back light unit, so that the manufacturing costs of the back light unit is reduced. The brightness difference exhibited on the display screen is also reduced, so that a brightness uniformity is achieved.