The method includes, while the article 1 is carried with the main portion 10 in contact with the carrying surface 20 and the to-be-folded portion 11 in contact with the folder plate 3A, 3B: a first guiding step, wherein the first guide surface 31 guides the to-be-folded portion 11 of the unfolded article 1, on a side of the carrying surface 20, in parallel to the carrying surface 20； a second guiding step, following the first guiding step, wherein the second guide surface 32, while being in contact with the to-be-folded portion 11, gradually raises the to-be-folded portion 11 while moving downstream； and a third guiding step, following the second guiding step, wherein the third guide surface 33 guides the to-be-folded portion 11 so that the to-be-folded portion 11 is folded onto the main portion 10.
