PURPOSE: To perform always satisfactory focus detection even if a focal length changes by providing an optical path splitting means in the optical path of the optical image-forming system of a zoom lens.

CONSTITUTION: An image-forming optical system is constituted of an image- forming lens 6 from a focusing lens 1 and an optical path splitting means 4 is provided on the optical path. Powers are reversely varied by a compensator 8 and a variator 9 cooperatively with zooming by a variator 2 and a compensator 3, whereby the focal length constituted of 1, 2, 3, 4, 8, 9, 10 forming an optical system for focus detection is made constant irrespectively of the zooming of the image-forming optical system. As a result, even if the zoom lens is zoomed to the telephoto end after the focus is detected at the wide angle end, the deviation in the depth of field is obviated.