A focus detecting system for detecting the focus state of a taking lens capable of changing a focal distance, in which an object light to be guided to an imaging element for image production by the taking lens is branched and captured by a plurality of imaging elements for focus state detection having an optical path length difference, and the focus state is detected on the basis of image signals acquired from the plurality of imaging elements for focus state detection, includes: a moving unit for moving the imaging elements for focus state detection back and forth in an optical axis direction; and an optical path length difference changing unit for changing the optical path length difference between the imaging elements for focus state detection by shifting a position of each of the imaging elements for focus state detection according to a focal distance of the taking lens using the moving unit.