A zoom lens is disclosed comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of negative refractive power, a fifth lens unit of positive refractive power and a sixth lens unit of negative refractive power, wherein, during zooming from the wide-angle end to the telephoto end, the air separations between the i-th and (i+1)st lens units are made to vary properly and the ratio of the focal length of the fourth lens unit to the longest focal length of the entire system is made to have a proper value. In particular, a zoom lens is disclosed wherein the second lens unit is made to decenter to stabilize the image.