1. An image capturing apparatus comprising: one imaging optical system; 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 said one imaging optical system; 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 said 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 said detection unit; and an extraction unit which is able to create an image for binocular stereopsis using an extraction image, extracted from a region which includes the given subject and has a predetermined size, for each of the left-eye captured image and the right-eye captured image, wherein said extraction unit sets the region so that the difference, between the left-eye captured image and the right-eye captured image, of a subject which is different from the given subject and included in both the extraction images has a predetermined value.