A method for modifying optical properties of a synthetic resin increases a transmittance by modifying, removing, or reducing factors which absorb wavelengths in an ultraviolet range, such that it is possible to maintain stable and excellent reliability even after a long-term use. In a method for modifying optical properties of a synthetic resin, a modification treatment which is an energy irradiation treatment is performed on a cured product made of a synthetic resin to increase a transmittance in an ultraviolet range. For example, the modification treatment which is at least one of a photo-oxidation treatment and a thermal oxidation treatment is performed on the cured product made of the synthetic resin to increase the transmittance.