PURPOSE: To permit adjust of spectral reflectivity to a set value by consisting the 1st layer on the surface of optical parts made of a synthetic resin of silicon monoxide, the 2nd layer of cerium dioxide and the 3rd layer of silicon dioxide.

CONSTITUTION: The 1st layer 2 of the antireflection films of the optical parts made of the synthetic resin by forming the three-layered structure consisting of the 1st layer, 2nd layer and 3rd layers successively from the surface side onto the surface of the optical parts made of the synthetic resin is formed of the silicon monoxide (SiO), the 2nd layer 3 of the cerium dioxide (CeO2) and the 3rd layer of the silicon dioxide (SiO2). SiO which has the high adhesive effect to the synthetic resin contributes to the prevention of cracks and the balanced constitution of SiO (compressive stress), CeO2 (tensile stress) and SiO2 (compressive stress) contributes to the prevention of the film destruction of the respective film layers or over the entire part of the three layers by the film stresses. The spectral reflectivity is adjustable by setting the refractive indices and thicknesses of the respective film layers. The generation of cracks is thereby obviated and ≤0.5% spectral reflectivity is substantially satisfied.