Optical system having a camera shake compensating function

A zoom lens system includes a compensating lens unit (GrA) capable of decentering for camera shake compensation and a non-decentering lens (N) located closer to an image than the compensating lens unit (GrA) and not decentering in camera shake compensation. An aspherical surface is formed in the compensating lens unit (GrA), and an aspherical surface which tends to counteract an aspherical effect of the aspherical surface formed in the compensating lens unit (GrA) is formed in the non-decentering lens (N). The following conditions are fulfilled: ω≦10°Df≦0.3 where ω is a half angle of view, D is a distance from an aperture stop (S) to a decentering lens unit, and f is a focal length of the entire lens system.