Abstract: A focus adjusting device for flange back adjustment of an inner focus lens assembly including a zoom lens, a focus lens, and a compensator lens, comprises: driving means for driving the zoom lens; zoom lens position detecting means for detecting information on position of the zoom lens; focus lens driving means for driving the focus lens; focal condition detecting means for detecting a focal condition; and computing means for computing an optical end of the focus lens by using information obtained previously on a ratio between the distance from one to the other of opposite mechanical ends of the zoom lens and the distance from one to the other of opposite optical ends of the zoom lens.