Provided is a planar light source device, which comprises a light guide having at least one side end serving as a light incident side and having one surface serving as a light-emitting side, a light source disposed in the vicinity of the side end of the light guide, and a light reflector disposed on the other side opposite to the light-emitting side, and which is characterized in that the light guide is essentially formed of a polyolefin resin, the light reflector is also essentially formed of a polyolefin resin, and when a piece of the light reflector having a face size of 1.5 cm1.5 cm is prepared, and it is pressed against the surface of the light guide with its reflective face being in contact with the surface of the light guide under a load of 135 g/cm² thereto, and, in that condition, when it is reciprocated 10 times on the surface of the light guide to a width of 5 cm every time at a speed of 2.5 cm/sec, then the surface of the light guide is not substantially scratched by it. The planar light source device is characterized in that the light guide therein is hardly scratched and the device is lightweight.