An object list LO in which information of each object included in a structured document has been collected in a list format is formed. Objects in which a distance in the vertical direction of a document is equal to a threshold value or less are included in one object group and the objects in the object group G are grouped as one group. After that, in the case where a length in the horizontal direction of each of circumscribed rectangles of two or more objects included in the object group G is equal to a length in the vertical direction or more and a length in the horizontal direction of at least one of the two or more objects is smaller than a threshold value, a block reforming process is executed. In the block reforming process, among the objects in the object group G, the objects in which the distance in the horizontal direction is equal to the threshold value or less are grouped as one object group GC.