PURPOSE: To obtain a small-sized lens system which is variable in focal length by providing an auxiliary lens system as to be mounted and dismounted to an optical system having negative, positive, negative, negative, positive, and positive refracting powers successively from an object side and constituting said system so as to satisfy the prescribed conditions.

CONSTITUTION: The auxiliary lens system which can be mounted or dismounted for changing the focal length of the entire system on the optical axis just before the object side of an aperture diaphragm is provided to a main lens system having, successively from the object side, a negative meniscus lens of which the convex face is directed to the object side, aperture diaphragm, a lens of which both lens faces directing the stronger refractive faces to the object side as compared to the image plaside are convex, a lens combined with a lens of which both lens faces are concave and a lens of which both faces faces are convex, and a lens of which both lens faces directing the strong refractive faces to the image plane side as compared to the object side are convex. The auxiliary lens system has the lens combined with a positive lens and a negative lens and a 23rd lens of which both lens faces are concave. Said system satisfies the conditions expressed by equations IWV.