An image pickup apparatus comprising: an image sensor that includes a plurality of pixels in a two-dimensional arrangement of rows and columns, wherein the plurality of pixels include a plurality of image pickup pixels, each corresponding to a predetermined color, and a plurality of pairs of pupil-divided focus detection pixels, wherein pixel signals from the pupil-divided focus detection pixels are configured to be usable for a phase difference focus detection; and a controller (121, 124) configured to successively read out lines of pixel signals from the image sensor for a frame, characterized in that the controller has at least an image mixture readout mode, in which the controller is configured to output pixel signals from both of the plurality of image pickup pixels and the plurality of pupil-divided focus detection pixels, and in the image mixture readout mode, in which pixel signals are to be mixed in a predetermined unit of a predetermined number of pixels, the controller is configured to mix pixel signals of the image pickup pixels corresponding to the same color, which are included in the predetermined unit, to produce mixed image pickup signals and thereby reduce a number of the image pickup signals to be output from the image sensor, and to output the mixed image pickup signals from the image sensor for displaying images, while the controller is configured to output each pixel signal of each of the pupil-divided focus detection pixels, which are included in the predetermined unit, from the image sensor for the phase difference focus detection without mixing with another pixel signal. an image sensor that includes a plurality of pixels in a two-dimensional arrangement of rows and columns, wherein the plurality of pixels include a plurality of image pickup pixels, each corresponding to a predetermined color, and a plurality of pairs of pupil-divided focus detection pixels, wherein pixel signals from the pupil-divided focus detection pixels are configured to be usable for a phase difference focus detection; and a controller (121, 124) configured to successively read out lines of pixel signals from the image sensor for a frame, characterized in that the controller has at least an image mixture readout mode, in which the controller is configured to output pixel signals from both of the plurality of image pickup pixels and the plurality of pupil-divided focus detection pixels, and in the image mixture readout mode, in which pixel signals are to be mixed in a predetermined unit of a predetermined number of pixels, the controller is configured to mix pixel signals of the image pickup pixels corresponding to the same color, which are included in the predetermined unit, to produce mixed image pickup signals and thereby reduce a number of the image pickup signals to be output from the image sensor, and to output the mixed image pickup signals from the image sensor for displaying images, while the controller is configured to output each pixel signal of each of the pupil-divided focus detection pixels, which are included in the predetermined unit, from the image sensor for the phase difference focus detection without mixing with another pixel signal.