1. An image capturing apparatus comprising: an image sensor which includes a left-eye light-receiving element and a right-eye light-receiving element, and outputs a left-eye captured image for left eye and a right-eye captured image for right eye upon receiving light beams having passed through one imaging optical system; and one or more processors configured to function as the following units: a detection unit which is able to detect a difference, between the left-eye captured image and the right-eye captured image, of each of subjects included in the left-eye captured image and the right-eye captured image; a control unit which is able to control a focusing unit of the imaging optical system to focus on a given subject included in the left-eye captured image and the right-eye captured image, based on the difference of the given subject between the left-eye captured image and the right-eye captured image detected by the detection unit; and a creation unit which is able to create an image for binocular stereopsis using the left-eye captured image and the right-eye captured image, wherein the creation unit creates the image for binocular stereopsis so that a difference, between the left-eye captured image and the right-eye captured image, of a subject which is different from the given subject and is included in both the left-eye captured image and the right-eye captured image has a predetermined value in the image for binocular stereopsis.