To solve such problems that a scratch is caused in a paper surface and a thin sheet breaks to improve versatility, and also to solve such problems that a folding position is shifted and a variation in folding position is generated for each sheet to improve folding quality.

When a sheet P is folded by pressing a movable sheet folding guide on a predetermined position of the sheet P carried into a conveyance path 3 and causing the sheet P to enter an interval between a pair of folding rollers 2 p, 2 m disposed along the conveyance path 3, after stopping the carried-in sheet P at a predetermined folding position Xs, a sheet guide 4 in which a roller shaft 4 s having a circular cross section is rotatably supported at a tip part, is moved in a crossing direction Fs relative to a sheet surface Pf to press the roller shaft 4 s on the predetermined position of the sheet P, and the sheet P is caused to enter the interval between the pair of rollers 2 p, 2 m to fold the sheet P.