PURPOSE: An electric advertising board and a surface light source device therefor are provided to be stably operated and improve the visibility of an advertisement panel at night even if an electric advertising board is exposed to moisture including rainwater, etc. CONSTITUTION: A surface light source device(110) comprises a light guide plate(111), a pair of light emitting device parts, a pair of waterproof bodies, a reflective plate(114), and a first waterproof layer(115). The light guide plate forms a plurality of grooves in a predetermined pattern and refracts or reflects incidental light. The light emitting device part arranges a plurality of light emitting devices on a flexible and string-shaped substrate and emits the light. The waterproof bodies hermetically seal the entrance of a channel and blocks moisture inflow to the channel. The reflective plate is arranged on the back side of the light guide plate and reflects the light emitted from the light guide plate to a front side. A first waterproof layer is formed between the light guide plate and the reflective plate, blocking moisture penetration.