Abstract: There is provided a surface light source device comprising upper and lower substrates adhered to each other in order to form a discharge space, a reflection layer and a lower fluorescent layer stacked on an upper surface of the lower substrate, and an upper fluorescent layer stacked on a lower surface of the upper substrate, wherein the reflection layer has a thickness of 40 to 120 ?m, the lower fluorescent layer has a thickness of 10 to 60 ?m, and the upper fluorescent layer has a thickness of 10 to 25 ?m, and the device further comprises a first ion shield layer interposed between the upper substrate and the upper fluorescent layer, for blocking Na+ ions from being eluted from the upper substrate.