Abstract: In a variable focal length optical system which has, in succession from an object side, a first lens unit G1 having a positive refractive power, a second lens unit G2 having a negative refractive power, a third lens unit G3 having a positive refractive power, a fourth lens unit G4 having a positive refractive power, and a fifth lens unit G5 having a negative refractive power, when the focal length is varied from the maximum wide-angle state to the maximum telephoto state, at least the fifth lens unit G5 moves toward the object side, so that a first variable air gap between the first lens unit G1 and the second lens unit G2 increases, a second variable air gap between the second lens unit G2 and the third lens unit G3 decreases, a third variable air gap between the third lens unit G3 and the fourth lens unit G4 increases, and a fourth variable air gap between the fourth lens unit G4 and the fifth lens unit G5 decreases, the second lens unit G2 at least has a negative sub lens unit G21 which is disposed at a po