An illumination device LK comprises a planar light guiding element 3 having a first face 3c and a second face 3d, which are substantially parallel to each other, and a predetermined number (equal to or greater than one) of light sources 2-1, 2-2 for emitting light from a light emitting side along a main light emission axis A. The planar light guiding element 3 has inserting means 3h for allowing insertion of at least one of the light sources 2-1, 2-2 separately so that the main light emission axis A is substantially parallel to the first face 3c and the second face 3d.