A planar light source device includes a light guide plate, with its upper surface and lower surface as a first surface and a second surface. At least one light strip is provided with a plurality of light-emitting-diodes (LEDs), and is located on a side of the light guide plate. The LEDs are disposed in the light guide plate to emit light; and a reflective plate is disposed outside the second surface to reflect light a diffusion plate is disposed outside the first surface to scatter light reflected by the reflective plate. When manufacturing the light guide plate, the light-emitting-diodes (LEDs) are placed in a mold in advance, the LEDs and light guide plate form integrally, thereby avoiding loss of light, and raising the efficiency of the planar light source device.