PURPOSE: To prevent the reduction of the luminance of light beams in a position distant from a light source by directing the light beams from the light source, which does not face a light emitting surface, to a photoconductive plate by a reflecting sheet stuck to the light emitting surface.

CONSTITUTION: A matte frosted prism worked face 2a is formed on the front face of a light emitting surface 2, and a reflecting sheet 10b is stuck to a slope 7b formed on the rear face, and a reflecting sheet 10a is stuck to a slope 7a formed on one side of the front face of the light emitting surface 2. A photoconductive plate 12 which is formed into a wedge shape to be thinner according as going away from a light source 4 is formed on a main body 1 on which the slope 7b is formed. The light beams emitted from the light source 4 is led toward the photoconductive plate 12 by the large slope and the small slope 7a formed on a part of the body 1 and the large reflecting sheet 10b and the small reflecting sheet 10a stuck to them and is efficiently diffused while repeating irregular reflection in the photoconductive plate 12, and the light of the light source 4 is furthermore diffused into a uniform surface luminescence by the prism worked face 2a formed on the front face of the light emitting surface 2.