A sheet transport device includes a transport path along which a sheet is transported in one direction; a sheet butting portion provided at one side of the transport path; a sheet rotating section; and a sheet butting section. When a short sheet is transported along the transport path, the sheet rotating section rotates the short sheet such that a leading end is directed to the other side of the transport path, and when a long sheet is transported, the sheet rotating section rotates the long sheet such that an angle of inclination with respect to the one direction is reduced. The sheet butting section moves the short sheet rotated by the sheet rotating section toward the one side to make the side edge of the short sheet butt against the sheet butting portion, while allowing the long sheet to pass therethrough without moving the long sheet toward the one side.