An image capturing apparatus comprises: an image sensor that includes a plurality of pixels, each including a plurality of photoelectric conversion elements; a readout unit that reads out a signal from a portion of the photoelectric conversion elements of each pixel as a first signal and reads out a sum of signals from the plurality of photoelectric conversion elements of each pixel as an image signal; a generation unit that generates a second signal for each pixel using the image signal and the first signal; and a calculation unit that calculates a moving amount of a focus lens for achieving an in-focus state based on a phase difference between the first signal and the second signal. The calculation unit performs the calculation without using a signal from a defective line.