In the device disclosed, an image formed by an optical system is scanned by an image sensor composed of independently arranged photoelectric transducing elements to produce an image scanning signal. A detecting circuit detects the variation of the brightness between the adjacent image elements as a measure of the sharpness of the image and the focus of the optical system. The focus detection accuracy is improved by non-linear transformation of the brightness variation signal with a non-linear transformation circuit that weighs the brightness variation signal on the basis of the signal level. A signal forming circuit detects the difference between the outputs of transducing elements and adds the differences to produce a comparator signal when a predetermined relationship is reached between the sharpness indication and the addition signal. An indicator indicates that the focusing condition cannot be discerned in response to the comparison signal and a control inhibits the output of the detection circuit to the indicator prior to the addition signal.