A sheet processing apparatus has a stapler for stapling a center of plural stacked sheets; a sheet folding mechanism for folding a set of the stapled sheets in half; a sheet recognizing portion for recognizing the number of sheets forming the set of sheets; a sheet transport mechanism for transporting the set of stapled sheets to a position where the center of the set of sheets is folded by the sheet folding mechanism; a transport distance determination table for associating the number of sheets with a distance by which the set of sheets is to be transported by the sheet transport mechanism; and a sheet transport control portion for transporting the set of sheets by a distance determined by the transport distance determination table according to the number of sheets through use of the sheet transport mechanism.