An auto focus unit, comprising a pursuit block, a scanning area setting block, a change area setting block, a permission block, and a lens driver, is provided. The pursuit block pursues the movements of a target object. The scanning area setting block designates a part of an object-area as a scanning area at first timing. After first timing, the scanning area setting block is able to reset the scanning area to the now location where the targeted object has moved. The change area setting block defines an area of a predetermined range as a change area at the first timing. The permission block orders the scanning area setting block to reset the scanning area when the location where the targeted object moved is outside of the change area. The lens driver orders a focus lens to move so that an optical image of the scanning area is focused on.