1. An image pickup apparatus, comprising: an optical imaging system for forming an object image; an image pickup device including a plurality of pixels that photoelectrically converts the object image; a defocus quantity calculation section 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 passes 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 defocus quantity calculated at the defocus quantity calculation section; and a thinning-out section for thinning out the 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 the object image.