To provide a surface light source device, having a high utilization factor of the light from the light source, realizing even higher brightness and radiating light more uniformly for the case that it is used mainly as a backlight for a liquid crystal display.

The device comprises a prism sheet 1 emitting incident light from the rear side out of its surface with a nearly vertical outgoing angle θ1, a straight linear light source 2 arranged on the lateral or rear side of the prism sheet 1 and a shaft type movable mirror 3 reflecting the light irradiated from the light source 2 to the rear surface of the prism sheet 1. In this case the direction of the reflected light is continuously changed with the movable mirror 3.