The focus detection accuracy of a focus detection device that employs an external AF sensor unit having a pair of line sensors that are each comprised of multiple unit line sensors is improved. The individual line sensors are arranged so that the center of the field of view of an imaging optical system at a predetermined subject distance, at which field-of-view adjustment for causing the field of view of the imaging optical system and the field of view of a focus-detection optical system to match has been carried out, corresponds to the center of one of the multiple unit line sensors included in the individual line sensors.