An auto focus unit, comprising a pursuit block, a lens driver, a detection block, and a controller, is provided. The pursuit block pursues the movements of an optical image of the target-object. The auto focus unit brings an optical image of the target-object into focus on a light-receiving surface. The target-object is desired to be brought into focus. The lens driver causes a focus lens to move so that an optical image of a scanning-area is brought into focus. The detection block detects the light intensity of an area including the scanning area. The controller causes the pursuit block to suspend a pursuit of the target-object when the light intensity detected by the detection block is lower than a predetermined threshold value.