An image pickup apparatus 101 includes a line sensor constituted by a plurality of photoelectric conversion elements and capable of performing a photoelectric conversion of a pair of object images formed by luminous flux from an image pickup optical system 102, focus detectors 105 and 118 which detect a focus state of the image pickup optical system based on a pair of image signals outputted from the line sensor, and a block dividing portion 118 which divides the line sensor into a plurality of photoelectric conversion element blocks to form a plurality of effective focus detection areas in an image taking region. The block dividing portion changes a pattern by which the line sensor is divided into the plurality of photoelectric conversion element blocks so that an arrangement of the effective focus detection areas in the image taking region is switched in accordance with an operation by a user.