As illustrated in FIG. 7A, since second side incident light IL2 having a large amount of light is reflected by the steeply inclined surface 72, second side reflected light RL2 having a large amount of light is obtained, and a high brightness portion BR1 corresponding to the position of the steeply inclined surface 72 appears in the first image information IG1. Further, as illustrated in FIG. 7B, since even first side incident light IL1 having a relatively small amount of light is reflected by the steeply inclined surface 72, first side reflected light RL1 having a relatively large amount of light is obtained, and a high brightness portion BR2 corresponding to the position of the steeply inclined surface 72 appears in the second image information IG2. Here, the brightness of the BR1 is larger than the brightness of the BR2. In the case of FIGS. 7A and 7B, in S200 of the flow of FIG. 8, since the brightness of the BR1 is larger than the brightness of the BR2, S200 is yes, and in S202, the specifying unit 48 of the processor 40 specifies a position α adjacent to a first side DirL of a first range 91 (see FIG. 7A) in the sub-scanning direction DM or a position α adjacent to a second side DirR of a second range 92 (see FIG. 7B) of the sub-scanning direction DM, as the position of the crease 70. In this manner, the position of the weak crease 70 in the sheet P is specified.