To enable a sheet to be folded in an accurate position with a simplified folding mechanism in performing folding processing on the sheet with a pair of folding rollers, a second transport path is disposed in a direction for crossing a first transport path for guiding a sheet from a carry-in portion to a carrying-out portion, and in the cross portion are disposed a folding roller pair for performing folding processing on a sheet, and a folding deflecting member for inserting a fold of the sheet in the nip portion. Then, the folding deflecting member is comprised of a driven roller that comes into press-contact with a roller periphery of the folding roller pair, and a shift member for shifting the driven roller from a waiting position to an actuation position, and by the operation of shifting the driven roller from the waiting position outside the path to the actuation position, the sheet front end portion is fed to the nip portion from an upstream-side guide path formed in the second transport path. At this point, the shift velocity of the driven roller is made higher than the velocity of the sheet sent from the first transport path.