Provided is a zoom lens system including a compact focusing lens unit and having a suppressed change in image magnification at the time of movement of the focusing lens unit. A time segment value corresponding to a duration of periodic load variations is obtained and the supply voltage is adjusted to maintain an observed phase angle for a supply voltage and current at a value corresponding to a target phase angle. The dissimilar materials have one or more of thermal, structural, and lattice mismatches. 28 or lower, that the duty ratio of the pulse frequency is controlled to be 0.