A focus adjustment method of a zoom lens system is provided for correcting a focal point shift caused by manufacturing error. The zoom lens system includes three movable lens groups in which at least two of said lens groups are moved to perform zooming to form an image onto an imaging device. In the zoom lens system, a finite number of focal length positions at which a focus adjustment is performed is provided. The lens group closest to the imaging device out of the three lens groups includes a focusing lens group. A position of the focusing lens group is controlled via pulses of a stepping motor. The focus adjustment method includes the combination of performing the above control of the focusing lens group to determine a position of the focusing lens group via pulses of the stepping motor and determining an amount of adjustment of the imaging device.