In the exemplary embodiment of the present disclosure, a position of a weak crease in a sheet is specified using an image reading apparatus. In the related art, it is possible to specify a position of a strong crease using an image reading apparatus, but it is difficult to specify a position of a weak crease. FIG. 9A is a diagram illustrating (i) how to read a sheet having a strong crease in the related art (a cross section of a sheet P and a platen glass 130 of the image reading apparatus) and (ii) image information IG obtained by reading. The image information IG illustrated in FIG. 9A and FIG. 9B to be described later is image information of a partial region of the sheet P in the direction along a crease 70. As illustrated in FIG. 9A, in the sheet P on which the strong crease is formed, a recess is present at the position of the crease 70. Two steeply inclined surfaces 72 are also present that form opposed wall surfaces of the recess. For example, as illustrated in FIG. 9A, when the sheet P is placed on the platen glass 130 of the image reading apparatus so that the recess of the crease 70 faces down and the crease 70 is in a direction intersecting a sub-scanning direction DM of the image reading apparatus (a moving direction of the illumination unit of the image reading apparatus), and the sheet P is read, two high brightness portions HBR corresponding to the two steeply inclined surfaces 72 appear in the image information IG obtained by reading. Therefore, a center position CP of the two high brightness portions HBR can be specified as a position of the crease 70.