An image pickup apparatus includes: an optical imaging system; an image pickup device; a defocus quantity calculation circuit for calculating a defocus quantity based on a phase difference between a plurality of signals for focus detection obtained from a plurality of pixels for focus detection that respectively receive a light flux that has passed through a different pupil region of the optical imaging system; a focusing section for driving the optical imaging system so as to achieve an in-focus state in accordance with the calculated defocus quantity; and a thinning-out circuit for thinning out a plurality of signals for focus detection that are not used for calculating the defocus quantity, based on an analysis result of a signal component of an object image.