Optical element having an antireflection film

An optical element having a plastic substrate and an antireflection film of a λ/4 - λ/2 - λ/4 or λ/4 - λ/4 - λ/2 - λ/4 type (λ = 500 nm) provided in that order on the plastic substrate, wherein the layer of λ/2 is a high-refraction equivalent film containing at least three layers and having a refractive index of from 1.80 to 2.40, and the even-numbered layer of the equivalent film is an SiO2 layer is provided. The optical element having an antireflection film on a plastic substrate has good heat resistance.