An optical element includes a first surface, a second surface positioned to face the first surface, and a plurality of reflecting surfaces arrayed in a first region defined by the first surface and the second surface, wherein the reflecting surfaces have a first length in a first direction vertical to the first surface and are arrayed at a pitch in a second direction perpendicular to the first direction, light incident on one of the first surface and the second surface is reflected by the reflecting surfaces toward the other surface, and predetermined parameters satisfy predetermined relational formulae representing conditions for ensuring total reflection at the reflecting surfaces and for avoiding total reflection at a light emergent surface.