1. An image capturing apparatus comprising: an image capturing unit configured to capture an image of a subject; an image capturing control unit configured to perform image capturing using a first pixel group of the image capturing unit during a first exposure time to obtain a high-exposure image, perform image capturing using a second pixel group of the image capturing unit during a second exposure time which is shorter than the first exposure time to obtain a first low-exposure image, perform image capturing using the second pixel group during a third exposure time which is shorter than the first exposure time to obtain a second low-exposure image, the second and third exposure times at least partially overlapping the first exposure time, and perform image capturing using the first pixel group during a fourth exposure time which is shorter than the first exposure time after the high-exposure image has been obtained, to obtain a third low-exposure image; a detection unit configured to compare the first low-exposure image with the second low-exposure image to detect a motion of the subject; and a generation unit configured to compose the high-exposure image with at least one of the first, second, and third low-exposure images, based on the motion of the subject detected by the detection unit, to generate a high-dynamic-range (HDR) image, wherein the generation unit, when the detection unit has not detected a motion of the subject, composes the high-exposure image with the third low-exposure image to generate the HDR image.