To provide a planar light source device of low cost and capable of firmly fixing an optical sheet and a light guide plate.

A frame 32 is provided with a housing part 32a surrounded by side walls 32b. Light sources 33 and a light guide plate 34 are stored inside the hosing part 32a, and a diffusion sheet 36 and prism sheets 37, 38 are superposed on the light guide plate 34. On a light-incident face 34a and an opposite face 34b of the light guide plate 34, firmly fixing faces 35 inflected in zigzag at either end part are set in recess. A portion of the light guide plate 34 and a portion of each sheet 36 to 38 are firmly fixed to the frame 32, by filling adhesive resin 46 in a gap between the firmly fixing faces 35 and the side walls 32b of the frame 32.