To miniaturize a processor, and to improve productivity by registering a succeeding sheet, in such a state that a sheet folded by folding rollers exists in a region where the sheet overlaps in the carrying direction with a part of a registering plate on the near downstream side of the folding rollers.

This sheet processor folds the sheet after forming an image, and has a pair of folding rollers, a registering means, a control means controlling the registering means, a thrusting-out means thrusting out the sheet registered to the pair of folding rollers, and a carrying means carrying the folded sheet. In registration, the sheet is registered by butting the registering plate reciprocatable in the sheet width direction against the end of the sheet. A part of its registering plate is positioned on the downstream side of the pair of folding rollers, and since the registering plate positioned on the downstream side has a shape of expanding in the sheet width direction, when a sheet P folded by the pair of folding rollers exists in an area of overlapping the folded sheet P in a cross section with a downstream part of the registering plate, a succeeding sheet P2 can be registered.