Local irregularities in luminance are removed from a surface light source device of side light type 20 applied to a liquid crystal display or the like. In assembly, a reflection sheet 4 and a primary light source 23 are mounted on a light guide plate 2, and these components are housed at a predetermined position in a frame 21. The primary light source 23 has a fluorescent lamp 25 and a reflector 26, and supplied illumination light to an end surface (an incidence surface) 2A of the light guide plate 2. The reflector 26 presses the reflection sheet 4 against the light guide plate 2 and supports the reflection sheet. A support member 24 has a claw 24C engageable with a claw 21C of the frame 21. A light absorbable layer 27 is formed on an end portion of the reflection sheet 4. Since the reflection sheet 4 projects toward the primary light source and is pressed against the light guide plate 2 by the reflector 26, there is less quantity of light actually incident onto a lower edge ED, even if the illumination light makes a propagation toward the edge ED. Thus, it is possible to prevent the phenomenon that the edge ED on the side of the reflection sheet 4 is brightly illuminated with illumination light from occurring in the vicinity of the end surface (the incidence surface) 2A of the light guide plate 2 to which the light is supplied.