To eliminate the occurrence of luminance unevenness and reduce the thickness by arranging a cylindrical light source, and forming a diffuse reflecting layer on a reflecting layer between it and the inner bottom part of a lamp house.

A diffuse reflecting layer 4 is preferably formed of a while material, particularly, a film-like sheet or plate material. The diffuse reflecting layer 4 can be also formed by coating a lamp house 3. As the white material used herein, polyester is preferable. According to this structure, the light emitted from the inner bottom side of the lamp house 3 of a cylindrical light source 1 is diffused and reflected by the diffuse reflecting layer 4, so that a back lighting device with little luminance unevenness can be provided. More specifically, a film-like diffuse reflecting layer 4 about 0.2 mm thick consisting of white polyester is arranged on the inner bottom part of the lamp house 3 having a reflecting layer on the inside bottom part, whereby the luminance unevenness is improved, and the thickness can be reduced about 2 mm.