PURPOSE: To eliminate the loss at the time of propagation of light at the boundary of a light transmission plate and to provide the panel light source which has high efficiency and excellent uniformity of brightness by laminating two sheets of diffusing materials varying in the content of the diffusing materials and limiting the content of one thereof, thereby forming the light transmission plate.

CONSTITUTION: The light transmission plate 1 contg. the diffusing materials formed by dispersing the diffusing materials into a transparent acrylic resin so as to have 70 to 95% transmissivity and the diffusing plate 2 formed by incorporating the diffusing materials b into the transparent acrylic resin so as to have 40 to 70% transmissivity are stuck to each other. A white reflecting sheet 3 having a high reflectivity is stuck to the rear surface of the light transmission plate 1. A fluorescent lamp 4 and a reflector 5 for making the light of this fluorescent lamp 4 efficient incident on the light transmission plate 1 from the end face thereof are provided along the end face of the light transmission plate 1. Both the front and rear surfaces of the light transmission plate 1 contg. the diffusing materials are made into a specular mirror state in this way and the loss at the time of light propagation at the boundary of the light transmission plate 1 is eliminated. The rate of utilizing the light is thus enhanced and the light emitting surface having uniform brightness is obtd.