1. An image-capturing apparatus comprising: an image-acquiring unit which acquires a first image that is captured by photographing an arbitrary place in a state where a paper sheet is laid on the arbitrary place, the arbitrary place including a contour other than that of the paper sheet, and a second image that is captured by photographing the arbitrary place in a state where the paper sheet is not laid on the arbitrary place; a difference image producing unit which produces a difference image between the first image and the second image acquired by the image-acquiring unit, thereby producing the difference image which includes contour information of the paper sheet and which does not include information on the contour other than that of the paper sheet; a contour extraction unit which extracts the contour information of the paper sheet from the difference image that is produced by the difference image producing unit, wherein the contour information extracted from the difference image is used for correcting a distortion of the first image; and an image conversion unit which is configured to correct the distortion of the first image based on the contour information that is extracted by the contour extraction unit so as to obtain an image of the paper sheet as if it were photographed from a frontal direction.