To provide a new method for efficiently dividing a polygonal region where internal angles in respective apexes become 90° or 270° into a plurality of rectangular regions while the occurrence of the rectangular regions including minute sides is suppressed.

In an attention side detector 52, an attention side is detected whose internal angle is 90° in the apexes positioned in both ends. A candidate rectangular region acquiring part 53 acquires a plurality of candidate rectangular regions where lengths of the adjacent sides are changed into a plurality of kinds while the attention side is set to be one side. A rectangular region deciding part 54 decides the rectangular region becoming an extraction object from a plurality of candidate rectangular regions while priority when the minute side occurs is made to be low, and priority when the minute disappears is made high in the remaining polygonal regions when the respective candidate rectangular regions are extracted from the polygonal region. Thus, the polygonal region can efficiently be divided into a plurality of rectangular regions while the occurrence of the rectangular region including the minute side is suppressed by repeating the processing on the remaining polygonal regions after the rectangular region is extracted.