A paper folding device having a function of alternately switching a bending direction to carry out folding continuously a plurality of times. The paper folding device comprises first and second pairs of folding roller pairs that bend a paper; a carry roller pair that feeds the paper to the first or second folding roller pair; and a biasing device for switching a direction in which the paper is bent by the first or second folding roller pair. The biasing device brings the paper into contact with one (lower side) folding roller of the folding roller pair on a side where the paper folds, and the paper to be folded is carried and folded. When paper is caused to enter a nip of the left folding roller pair by the left shifting device, tension is applied to the paper by a right shifting device. Thus, the angle at which the paper winds onto upper side folding rollers is adjusted.
