A self-scanning type photoelectric conversion element drive device, particularly, such a drive device applied to a focus detecting device of a camera, in which the effective dynamic range of the device is made very broad without a complex circuit arrangement. First and second photoelectric element arrays are scanned serially in the case that a sensed light intensity is below a predetermined value and scanned in parallel when the light intensity exceeds the predetermined value. For serial scanning, the outputs of the photoelectric elements are applied directly to an output terminal. In the case of parallel scanning, the output of one of the arrays is stored in a memory and then applied to the output terminal.