To provide a diffraction grating consisting of a specified cyclic hydrocarbon polymerized material.

A lightweight diffraction grating excellent in the optical characteristics and heat resistance and having low water absorption is obtained by this invention. Since the grating can be easily produced by an injection forming method or the like, excellent diffraction gratings can be produced at a low cost. Moreover, the diffraction grating can be preferably used for optical purposes such as a spectrometer, laser and optical pickup.