PURPOSE: To enable the range of variable focal lengths to be shifted as well by interchangeably mounting plural relay lenses to the image side of the zoom part including at least two lens groups and inserting another lens group in the relay lenses.

CONSTITUTION: The system is so constituted that plural relay lenses 11, 12 of mutually varying focal lengths are interchangeably provided to the image side of the zoom part 10 including at least two movable lens groups and that the same variable power lens 13 may be inserted to the zoom part 10 and each of the relay lenses 11, 12 having been mounted without changing respective back focuses.