a focus detecting apparatus including an optical system having a group of focusing lenses , an object distance measuring device which measures the amount of defocus for a specific object imaged by the focusing optical system , a focus judging device which judges the focus state in accordance with the measurement of the object distance measuring device and a lens driver for driving the focusing lens group to a focal position , in accordance with the amount of defocus , wherein the apparatus includes a controller for repeatedly operating the object distance measuring device and the lens driver , and a moving object judging device for judging that the specific object is a moving object when the focus judging device judges the out - of - focus state more than one time during the repeated operations .