<p>A light guide plate of a surface light source device of edge-light type in which a light source is disposed on a side surface of the plate. The light guide is formed of a transparent thermoplastic resin containing particles. The front surface of the light guide plate serves as a light output surface, and the back surface of the light guide plate is subjected to a light diffusion processing to have a gradation from the side surface toward the direction far from the light source. The light guide plate is characterized in that the color tone variation (?YImax-min) which is the difference between the maximum value (YImax) and the minimum value (YImin) of the yellow index (YI) determined from the tristimulus values (XYZ) in the 25 luminance measurement point on the light output surface is 20 or less.</p>
