1. An image capturing apparatus comprising: a distance measuring unit that measures a distance from a lens of a monocular camera for capturing an image of an object to one or more arbitrary points on the object; a diffuse reflection coefficient calculating unit that calculates a diffuse reflection coefficient of the object using the distance from the lens of the camera to a calibration image, a luminance of the calibration image, the distance to one or more arbitrary points on the object, which is measured by the distance measuring unit, and a luminance that is obtained by applying diffuse light to each of the one or more arbitrary points and that represents an intensity of brightness, which is estimated from a pixel value corresponding to a reflection light intensity representing an intensity of light reflected from the object, the calibration image being previously captured and serving as a reference when a distance to the object is calculated; and a distance calculating unit that calculates a distance from the lens of the camera to the object based on the distance from the lens of the camera to the calibration image, the luminance of the calibration image, the diffuse reflection coefficient of the object, which is calculated by the diffuse reflection coefficient calculating unit, and the luminance of the object.