A thin planar illumination device with a high light utilization efficiency is provided. In a planar illumination device 10, light source sections 15, 16, 17 and 18 each radiates laser light. The light guide members 19 and 20 polarize, by total reflection, light incident from side surfaces 19b and 20b on two opposite sides thereof, so as to be radiated from incident surfaces 19a and 20a, respectively. In a light guide plate 21, the light is incident from side surfaces beside the respective light guide members 19 and 20 so as to radiate irradiation light 24 from main surfaces 21a and 21b. The main surface of the light guide plate 21 is formed by a plurality of inclined surfaces, each being gradually thinned from the edge surfaces on two sides thereof toward a center thereof.