PURPOSE: To prevent the natural movement of a moving lens group by the own weight by providing a means for applying the force to serve as a load on the rotating motion of a movable barrel and making the means disconnectable.

CONSTITUTION: The forward/backward revolutions of a motor 22 are transmitted with gears 25, 35 to the gear 23 of an operation ring 6 to move a moving lens group 8 in an optional direction while an idle gear 35 is held intermeshed with an operation gear. If the ring 6 is fixed by a certain cause, the motor gear 25 begins to slip relatively with a motor shaft 24 by the effect of a wave washer 26 and a nut 27 fixing the gear 25 so that the rotating torque of a specified level or above is not exerted on the motor 22. If a clutch knob 38 is rotated, a clutch cam 39 rotates so that the tapered part 30 of a change-over member 29 fits into the circular conical part 31 provided to a stationary shaft 28.