PURPOSE: To easily change the focal length without extending the overall length of a lens system by extending the first and second groups to the object side within the range of the air interval between the first group and the second groups in the wide angle end and setting an auxiliary lens system in the image surface side on the optical axis of the second group with respect to a zoom position in the telephoto end.

CONSTITUTION: The first group I having a negative refracting power and the second group II having a positive refracting power which are arranged in order from the object side are provided; and when the second group II is moved to the object side to vary the power from the wide angle side to the telephoto side, the position on the optical axis of the first group I in the telephoto end is nearer to the image surface side in comparison with that in the wide angle end, and the first group I and the second group II are moved to the object side within the range of the air interval between the first group I and the second group II in the wide angle end and an auxiliary lens system C is set in the image surface side on the optical axis of the second group II with respect to the zoom position of the telephoto end, thereby changing the focal length of the whole system. Thus, the focal length in a normal variable power range is easily and quickly switched to that of another without extending the overall length of the lens system.