To eliminate a defect that when paper is folded up manually, the folding widths may become uneven or bending may be caused, thus preventing the paper from being folded up finely.

When paper is folded up using a paper folding auxiliary tool, one edge of the paper folding auxiliary tool is formed in a straight line, also two points in which a distance from this straight-line edge is substantially equal to a given folding width are connected by a convex pull-out groove in a direction of the straight-line edge, an edge of paper is pinched between a base of the paper folding auxiliary tool and an upper face of a tongue-like part enclosed by the convex pull-out groove, the edge of the paper is applied to both ends of the convex pull-out groove, also a part in which the paper protrudes from the straight-line edge of the paper folding auxiliary tool is turned back by 180 degrees along the straight-line edge of the paper folding auxiliary tool, and the paper is folded up with a given folding width.