A planar light source of this invention is formed with chamfered portions at an upper end of the light receiving side surface where the light receiving side surface and the light emitting surface of the light guide plate intersect and at a lower end of the light receiving side surface where the light receiving side surface and the reflection surface of the light guide plate intersect. The chamfered portions can make indistinguishable the bright lines, that may be produced during illumination, to prevent a dark-bright fringe pattern from showing up, thereby preventing a possible degradation of liquid crystal display quality.