To provide a planar lighting device wherein the efficiency of using light is high, and a light guide plate and a light source are not damaged by expansion and contraction of the light guide plate, even when the system is made large, thin, and light in weight.

The planar lighting device includes the light guide plate having a rectangular light-emitting surface and a light-entering surface including one side of the light-emitting surface, the light source disposed face to face with the light-entering surface, and a protective sheet disposed between the light-emitting surface of the guide plate and the light source.