Paper bumps against the 1st stopper [A] to make an arch in the paper near the 1st fold nip [B]. With this arch, the paper is fed into the 1st fold nip [B] and the 1st fold is formed. After that, the paper bumps against the 2nd stopper [C] and the 2nd fold is made at the 2nd fold nip [D]. After passing through the 2nd fold nip, the paper bumps against the 3rd stopper [E] and the 3rd fold is made at the 3rd fold nip [F] and the double gate-fold is formed.