An optical element of the present invention includes: a primary surface-1 whereon diffuse illuminating light radiated by a light source device is incident; a primary surface-2 formed in a concave cross-sectional shape when viewed from the primary surface-1, which reflects only illuminating light incident from the primary surface-1; a primary surface-3 formed in a convex cross-sectional shape when viewed from the primary surface-1, which reflects only diffuse illuminating light radiated by the light source device; and a primary surface-4 which radiates illuminating light incident from the primary surface-1 and illuminating light reflected by the primary surface-2. The primary surfaces-1 through -4 are located at positions such that the light beams reflected by the primary surface-2 and the light beams reflected by the primary surface-3 are substantially parallel.