The optical element includes a base member, and a first layer which is formed on the base member and whose refractive index for a central use wavelength λ changes in a thickness direction of the first layer by 0.05 or more. The first layer has an anti-reflection function and satisfies nt=ni+0.1(ns−ni), 0.5≦[n{t(nt)/2}−nt]/[ns−n{t(nt)/2}]≦0.8, λ/4≦t(nt)≦2λ and 1.0≦ni≦1.1. ni represents a refractive index of a most light entrance side part of the first layer for the central use wavelength, ns represents a refractive index of a most base member side part of the first layer for the central use wavelength, t(nt) represents an optical film thickness of the first layer at which the refractive index thereof for the central use wavelength is nt, and n{t(nt)/2} represents a refractive index of the first layer for the central use wavelength at a position where the optical film thickness ism t(nt)/2.