To provide a surface light source device in which spot light sources are turned into a plane light source, whereby a uniform intra-plane brightness is accomplished.

Photo-guide elements 3 as a means to turn the light from an LED 2 into a linear light source are installed along a light incident end face of a photo-guide plate 4 which turns the light into a surface light source. In the photo-guide elements 3, part of the light projected from the LED 2 is put in repetitive reflection, conducted in the direction parallel with the incident edge member 4b as turned into a linear light source, and is coupled with the light incident end face 4a of the plate 4. Thereby the intensity of the incident light at the end face 4a is made uniform, and a uniform intra-plane luminance can be achieved.