PURPOSE: To minimize the focusing positions of a fourth lens group in a wide angel end, a telescopic end, and the middle point, and thus reduce the slippage of the fourth lens group focusing positions over the whole area extending from the wide angel end to the telescopic end by conducting focus adjustment in three points of the wide angle end, the telescopic end, and the middle point.

CONSTITUTION: A second lens group is set in three points zp1-3 from a wide angle side to a telescopic side. The second lens group position zp1 and the focusing position fp1 of a fourth lens group are assumed as a wide angle end and the standard position of the fourth lens group, respectively, and the telescopic end vicinity is determined so that the moving orbit of the fourth lens group on design passes the fourth lens group position fp3. The difference Δf2 between the focusing position fp2 of the fourth lens group in the set second lens group position zp2 and the fourth lens group position on design in this position is calculated. The standard position fb of the fourth lens group is moved by this difference Δf2. The wide angle end position and the telescopic end position are calculated so that the moving orbit of the fourth lens group on design passes the fourth lens group positions fp1 and fp3.