To prevent a diffusion plate from warping, while maintaining lightweight and light emitting quality.

A direct backlight has a lamp 3 located on the back 8 side of a diffusion plate 4, the front 14 of which is an emitting surface, where a rib section 10 with a screen structure is formed on at least either of the front 14 and the back 8 of the diffusion plate 4, recess parts 12 are formed in areas corresponding meshes of the rib section 10 so that the thickness of the recess parts 12 is less than the rib section 10, and the mesh size of the rib section 10 is minute to a degree that unevenness of light emitting is not caused substantially by the difference between the thickness of the diffusion plate 4 at the recess parts 12 and the thickness of the diffusion plate 4 at the rib section 10.