An optical system of a telephoto type is disclosed, in which a refractive optical element made of a solid material that satisfies.nu.d<30, .theta.gd<-3.333.times.10.sup.-3.nu.d+1.40 and .theta.gF<-2.615.times.10.sup.-3.nu.d+0.67 is installed, .nu.d representing an Abbe's number, and .theta.gd and .theta.gF representing partial dispersion ratios. A refractive power with adequate sign is given to the refractive optical element in accordance with its location. Thereby, an optical system, which can excellently correct various aberrations such as chromatic aberration, and can be manufactured easily, and has an excellent environment resistance, is realized.