Among a plurality of lens units constituting an optical system, at least one of the plurality of lens units includes a refractive optical element made of a solid material that satisfies the following conditions: 3<.nu.d<30 0.10<.theta.gd<1.25 0.10<.theta.gF<0.60 where .nu.d is an Abbe number of the solid material and .theta.gd and .theta.gF represent partial dispersion ratios of the solid material. The refractive optical element made of the solid material has refractive power with a sign opposite to a sign of refractive power of the lens unit including the refractive optical element.