To determine an appropriate focus detecting process in a case where a relative attitude state between an optical system and an image pickup element is deviated from a reference attitude state.

This focus detecting device according to the present invention includes: an imaging optical system; a sensor chip disposed with focus detecting pixels that receive a pair of light fluxes passing through a pair of regions of an exit pupil of the imaging optical system and output a pair of image data corresponding to a pair of images formed by the pair of light fluxes; focus detecting means that calculates a phase difference between a pair of image data and detects a focus adjustment state of the imaging optical system based on the calculated phase difference; and determination means that determines a detecting process of a focus adjustment state by the focus detecting means based on attitude information representing a deviation amount from the prescribed arrangement of the imaging optical system and the sensor chip.