PURPOSE: To obtain the titled lens system which is compact and has the exit pupil position far from a focal plane by constituting an auxiliary optical system having 2 lens groups having positive and negative refracting powers successively from an object side in such a manner that said system satisfies the prescribed conditions.

CONSTITUTION: The auxiliary lens system having a focus detecting element 7 at its focal plane is separately provided on a photographing lens system 1 and the auxiliary lens system is set at a focusing state by extending part 5 of the auxiliary lens system cooperatively with the extension of a focusing lens group 4 of the lens system 1 for the purpose of focusing. Said system consists, successively from the object side, of the movable 1st lens component having the positive refracting power and the stationary 2nd lens component having the negative refracting power. The 2nd component consists, successively from the object side, of a negative single lens and a positive single lens spaced therefrom. The 2nd lens component satisfies the conditions expressed by equation I and equation II. In equations, f1: the focal length of the 1st lens component, d2: the air spacing between the negative and positive lenses in the 2nd lens component, νij: the Abbe number of the i-th lens material from the object side in the i-th lens component.