An optical element irradiated with a light flux having a wavelength of 350 nm to 450 nm, is provided with a synthetic resin base material; and an antireflection film provided on the synthetic resin base material and including a contact layer being in contact with the synthetic resin base material, wherein the contact layer is a film made of a material containing no oxygen atom or a film made of a mixed material or a material containing no oxygen atom and a material containing an oxygen atom in which the mixed material contains the material containing an oxygen atom less than 10%.