A planar light source device includes a cold cathode fluorescent tube, a light guide opposed to the light source to emit light which is received from the light source, a frame which supports the light source and the light guide and has a projection which positions the light source and the light guide, and an optical sheet which provides an optical property for light radiated from the light source and has a regulation section that regulates contact between the light guide and the projection of the frame, the regulation section being located between the light guide and the projection.