Disclosed is a light reflector formed of a white polyolefin film which contains a polyolefin resin and a filler and is oriented at least monoaxially to have an areal draw ratio of from 1.3 to 80 times, which has a whole ray reflectance of at least 95% and undergoes a dimensional change of at most 1.5% when left at 70.degree. C. for 300 hours, and which has a melting-starting temperature of 70.degree. C. or higher. The light reflector hardly causes brightness unevenness of the planar light source when it is built in a planar light source device and used for a long period of time.