To fully open the pages of a book and to completely copy it by inserting a slender guard to the residue generated by the insufficient length of a document at a binding part, perforating a plurality of string holes passed through a flat surface along the back surface side edge of the binding part, and inserting a flexible string.

Documents 2 to 4 having different sizes and thicknesses are aligned at the ends, and the page numbers are matched to a predetermined position. In this case, since the thickness difference occur between large and small documents 2 and 3, 4, the slender cardboard is cut in responsive to the length of the upper ends of the small documents 3, 4 from the upper end of the document 2 in response to the thickness of the documents 3, 4 to form a guard, which is mounted at the residue of the binding part 8 to eliminate the difference of the thicknesses. Then, back covers 11 are disposed on both upper and lower surfaces, the back side of the part 8 is fixed with paste 12 to form a bookbinding body. A plurality of string holes 9 are perforated at from the flat surface to the lower surface of the part 8, a flexible string 10 is inserted to the hole to bind the rear covers 11, 12 at the outside.