An optical element having a laser damage suppression film which is capable of withstanding high power blue laser and formed on a plastic substrate. Specifically disclosed is an antireflection optical element wherein a multilayer film, which is composed of low refractive index material layers (105) and high refractive index material layers (107) arranged alternately, is formed on a plastic substrate. In this antireflection optical element, the oxygen permeability coefficient is decreased such that variation in laser permeability is 2% or less after being continuously irradiated with blue laser having an energy density of 120 mW/mm2 for 1000 hours at an ambient temperature of 25° C. In one embodiment, the oxygen permeability coefficient is set at 30 cm3·mm m2·24 hr·atm) or less.