An optical element is constituted of a polygonal column-shaped transparent body which has four inner wall surfaces that surround a rectangular column-shaped space having a square-shaped opening when seen in a plan view, four outer wall surfaces that are respectively parallel to the inner wall surfaces, and two outer wall surfaces which are perpendicular to a diagonal line of the square-shaped opening and face each other, in which the distance between the two outer wall surfaces facing each other is longer than the diagonal line.