PURPOSE: To reduce a rate of light that the light radiated from a cylindrical light source is reflected by a cylindrical light source reflection body, and is returned to again a cold cathode tube and absorbed into the surface and to enhance incident efficiency to a transparent light transmission body by specifying the positional relations of the cylindrical light source and the cylindrical light source reflection body.

CONSTITUTION: A problem that light from a cylindrical light source is made incident on a transparent light transmission body efficiently in any condition when the thickness of the transparent light transmission body, the tube size of the cylindrical light source, the shape of the cylindrical light source reflection body and the positional relations among them are made to parameters respectively is dissolved by simulation by a personal computer. Then, the positional relations among the transparent light transmission body 1, the cylindrical light source 2 and the cylindrical light source reflection body 3 are made as follows. That is, the incident efficiency to the transparent light transmission body 1 is enhanced by making a distance X from a transparent light transmission body incident surface to a cylindrical light source 1.2mm or below. Thus, the light quantity that the light reflected by the cylindrical light source body 3 is absorbed into the surface of the cylindrical light source 2 is reduced.