A paper folding device is configured so that a folding line is always formed at a predetermined position of paper sheets. A paper supply section is provided with a conveyor belt for conveying paper sheets (P) sequentially supplied from a paper sheet stack. A guide plate (4) for aligning the positions of the conveyed paper sheets is provided on a conveying surface of the conveyor belt. A paper folding section has a buckle (10) comprising an outer frame (11) and bars (12) which extend between opposite sides (11a, 11b) of the outer frame and are arranged at intervals from each other. Paper sheet introducing gaps extended in the direction of the length of the bars are provided in the buckle. Before the start of paper folding operation, a control section calculates and processes data relating to the size of paper sheets and data relating to the position of the bars, so that the distance to the guide plate from a reference point in the direction perpendicular to the conveying direction (R) is set to a value with which a side edge of each paper sheet introduced into the paper sheet introducing gaps in the buckle is separated by a distance not less than a predetermined level from a side edge of a bar located closest to the side edge of the paper sheet. Then, the control section sets the position of the guide plate according to the set value.