into the switchback path 22 downstream at the point where the leading edge of the sheet is conveyed a predetermined amount downstream the switchback roller 22 f is driven in reverse while the conveyance roller 25 is stopped at that point the trailing edge of the sheet is restrained by the conveyance roller 25 and the center of the sheet is bowed in the direction of the nipping point np 1 between the first and second rollers 21 a and 21 b the sheet is nipped between the both rollers and the first folding process is executed therebetween by adjusting the distance between amount of feeding of the switchback rollers 22 f and the nipping point n 2 the outside folding position n in fig4 b c is set specifically the sheet sensory detects the leading edge of the sheet the control cpu 59 rotates the switchback roller 22 f in reverse after an estimated amount of time after that signal for the sheet folding position n 2 to reach the nipping point np 1 at that time the leading edge of the sheet is folded between the second rollers 21 a 21 b at the folding position n 2 facing outward in this way the sheet folded to face outside is fed to the folded sheet path 23 by the first and second rollers 21 a 21 b at this time the cpu 59 moves the movable stopper 23 s using a drive motor not.