A planar light source device capable of increasing light use efficiency and preventing color breakup without using any reflection polarizing plate nor performing multiple reflection. The planar light source device includes a planar light guide plate, light sources arranged as to be opposed to end surfaces of the light guide plate on its both sides, and a divided prism formed in a back surface inside the light guide plate and that reflects light incident upon the light guide plate from the light sources toward a front surface side of the light guide plate. The divided prism includes a prism portion and a dividing portion both of which have a width equal to or less than the wavelength of light from the light sources and is formed with the prism portion and the dividing portion alternately repeated along a direction parallel to the end surfaces of the light guide plate which are opposed to the light sources.