An optical element having an antireflection effect includes an incident plane and an emission plane and a conical microstructure formed on at least one plane of the incident plane and the emission plane. The conical microstructure includes a first structure formed of a plurality of conical microprojections with an average height H1 and a second structure formed of a plurality of conical microrecesses with an average height H2. The average height H1 satisfies an inequality H1≦1/3*λ/n and the average height H2 satisfies an inequality H2≦1/3*λ/n. An average pitch P between the first structure and the second structure satisfies following conditions: H1/2≦P≦1/3*λ/n, and H2/2≦P≦1/3*λ/n, in which the use wavelength is defined as X and the refractive index relative to the use wavelength is n.