Based on layout information, in regard to a group of reusable objects appearing in a range from a lowermost layer to a layer immediately before a variable object appears in the order of arranging image objects, one new reusable object is generated for each group of pages on which reusable objects constituting the group of reusable objects are identical and have same arrangement positions, the new reusable object generated by merging the reusable objects constituting the group of reusable objects. Also new layout information is generated by replacing description of each of the reusable objects constituting the group of reusable objects in the layout information with description of the new reusable object corresponding to the group of reusable objects.