To provide an image capturing device which performs generation of a display image and detection of phase difference using image signals read from a read-out region of an image sensor and is capable of preventing reduction in accuracy of focus detection which is done using the detected phase difference.

An image capturing device includes an image sensor with pixel units, each having a plurality of photo-electric conversion sections for one microlens. The image capturing device defines a read-out region for reading from the pixel units, detects phase difference from image signals read from the region, and outputs reliability of the phase difference. The image capturing device assesses validity of the phase difference detection using the phase difference reliability. If the phase difference detection is valid, the image capturing device carries out a focus adjustment process using the detected phase difference and, if the phase difference detection is invalid, shifts the position of the read-out region without changing a position of a region, located inside the read-out region, used for generating a display image.