PURPOSE: To move complicated lens groups with a small number of frame constitutions by moving a focusing groove to secure intervals of lens groups required for zooming.

CONSTITUTION: When a power varying system lens holding frame 2 is moved, the extent of movement of this frame 2 is detected by a detecting means 7. The extent of movement of a focusing system lens holding frame 1 is determined by a stored value from a storage device 9 corresponding to the detected extent of movement, and the focusing system lens holding frame 1 is moved for said extent of movement in the direction of the optical axis with respect to the power varying system lens holding frame 2 by a control means 8. Thus, a zoom mechanism is obtained where many lens groups are moved in the simple constitution.