PURPOSE: To efficiently utilize a motor which has small output torque by fitting a hollow motor to a lens barrel coaxially, providing a differential gear type speed change gear to the lens barrel, and reducing the speed of the rotation of the hollow motor by the speed change gear and driving a focus ring.

CONSTITUTION: An external gear 25, an internal gear 26, and a planetary gear 27 constitute a differential gear device and the external gear 25 which constitutes a sun gear is driven by the motor 21. Namely, when the coil 22 of the motor 21 is powered on, the rotor 23 is driven to rotate the sun gear 25, whose rotation is reduced by the differential gear type speed change gear and transmitted to the focus ring 18. Then the focus ring 18 is supported on the lens barrel 15 by a helicoid 19, so axial motion is performed by the rotation to perform automatic focusing operation. Consequently, a large-torque motor is not necessary and the size of a magnet is reduced to enable efficient utilization.