the planar light source of this invention comprises : a light guide plate made of a platelike , light - transmitting material formed with a reflection surface on a bottom side thereof , the reflection surface being formed by combining a plurality of kinds of reflection means ; and an illuminating light source arranged opposite a light receiving side surface of the light guide plate ; wherein the light guide plate has formed smooth a boundary on the reflection surface between the plurality of kinds of reflection means so as not to form any step between the adjoining reflection means . this construction prevents the formation of a dark - bright fringe pattern between the plurality of reflections means when the light source is illuminated .