When a zoom lens system is focused by moving a lens other than the foremost lens group, the advancing amount of the focusing lens varies according to the variation in the focal length even whyen focusing on an object at the same object distance. When a zoom lens system is zoomed to a certain focal length, the object distance and advancing amount of the focusing lens has a relationship expressed by a function resembling a logarithmic function. The present invention enables one to focus a zoom lens system using only one movement amount curve by arranging to move the focusing lens along the movement amount curve, utilizing different portions of the movement amount curve according to the amount of zooming.