To provide a surface light source device of which the thickness can be reduced by preventing generation of moire fringes, and to provide a liquid crystal display device including it.

In the surface light source device 20, light emitted from a light source 1 passes through the inside of a light guide plate 2, and an irregular pattern 3 formed at a predetermined pitch reflects the light to be emitted from a light emission surface 2a of the light guide plate 2 in a predetermined direction. A reflecting plate 4 reflects light, which leaks from the light guide plate 2 to the side opposite to the light emission surface 2a, toward the light guide plate 2. Because an optical branching layer 6 partly transmits and partly reflects light emitted from the light emission surface 2a in a predetermined direction, light with an emission pattern of which the unevenness due to the pitches of the irregular pattern 3 is smoothed is incident on a prism sheet 5 even if the light guide plate 2 is thin. The prism sheet 5 corrects the incident light so as to be in the normal direction of the light guide plate and emits the light.

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