To solve a problem that it is difficult to neatly perform the tri-folding of a sheet by measurement by sight, although the tri-folding is frequently performed in office business or the like and there is also a method of folding after placing a mark, it takes a time and effort and the mark is conspicuous where, although a tri-folding dedicated machine is on sale, it is expensive, and there is no person other than a person in charge of massive tri-folding may not easily purchase it, and where, although an inexpensive instrument for the tri-folding is proposed, the folding is performed in conformity with the instrument or the folding is performed together with the instrument, and attention should be paid to the shift of a sheet during work, and the shift is liable to occur, and careful work while paying attention to the shift cannot quickly and easily perform the tri-folding.

Folding is performed by this novel method where, when a sheet is set on this instrument, a position at 1/3 the sheet becomes a corner of the instrument. The corner of the instrument is followed from above the sheet, and a folding line is formed on the sheet. A step is provided for stopping paper, and the sheet setting is made smooth. By providing a slip stop to a near place 5-6 mm from the corner of the instrument, the shift of the sheet during work can be eliminated.