A zoom lens system includes at least three lens groups. Zooming is performed by moving at least two lens groups. The first lens group and the second lens group are formed as the focus-adjusting lens groups which are movable in the optical-axis direction for performing the narrower zoom adjustment when the zoom lens system is being assembled. The focus-adjusting lens groups satisfy the following condition:

wherein

• K1(L) designates the focus sensitivity of the first lens group at the long focal length extremity;
• K1(S) designates the focus sensitivity of the first lens group at the short focal length extremity;
• K2(L) designates the focus sensitivity of the second lens group at the long focal length extremity; and
• K2(S) designates the focus sensitivity of the second lens group at the short focal length extremity.