An optical system includes at least one of a movable lens unit movable in an optical axis direction and a fixed lens unit that is not movable in the optical axis direction. The at least one lens unit includes a plurality of refractive optical elements. The refractive optical elements are made of resin and have focal lengths of the same sign. When .phi.i is a refractive power, with respect to d-line, of the i.sup.th refractive optical element from an object side, .phi.G is a refractive power of a negative lens in the lens unit including the refractive optical elements, and .phi.a is an average refractive power, with respect to d-line, of the refractive optical elements, .phi.i, .phi.G, and .phi.a are adequately set.