An image processor according to the present invention includes a reference information acquisition unit that acquires reference information for specifying a reference source and a reference target associated with the reference source from an electronic document having the reference information indicated therein, and a print instruction information production unit that produces print instruction information including an instruction for causing a printing device to print a reference source index for representing a reference source in a margin region located at a peripheral edge of a reference source page specified based on the reference information, and an instruction for causing the printing device to print a reference target index for representing a reference target in a region overlapping the region where the reference source index is printed in a margin region located at a peripheral edge of a reference target page specified based on the reference information.