A planar light illumination device contains: a light guiding plate, an ambient light sensor (ALS), and a controller. The light guiding plate includes a first light emitting unit and a second light emitting unit. The first light emitting unit has a first light emitter and a second light emitter, a color temperature of which is different from the first light emitter, and the second light emitting unit has a third light emitter and a fourth light emitter, a color temperature of which is different from the third light emitter. The ALS senses a color temperature of ambient lights, transforms the color temperature into a first color-temperature signal, and transmits the first color-temperature signal. The controller is electrically connected with the first and second light emitters of the first light emitting unit, the third and fourth light emitters of the second light emitting unit, and the ALS.