one side including the corner is gripped by the clip 45 on the right side position a in fig1 the clamp 35 gripping the sheet 1 rises in the direction of arrow b through the drive conveyor rail 31 at the uppermost point the clamp 35 transfers to a free conveyor rail 32 inclined slightly downward to lower in the direction of arrow c and temporarily stops at the position of lower end d a transverse transfer device 61 intermittently transfers clamp cases 62 at certain intervals when the clamp case 62 stops at position d it is possible to transfer the clamp 35 from the conveyor rail 32 to the clamp case 62 one or a plurality of clamps 35 are pushed into the clamp case 62 by a sequential feeding device not shown for feeding the clamp 35 with an air cylinder then the transverse transfer device 61 intermittently transfers clamp cases 62 the clamp case 62 containing clamps 35 reaches position e and stops there then an air cylinder 63 operates to push a clamp girder 42 of the clamp 35 which is in the clamp case 62 and grips the sheet 1 to push out the clamp 35 the clamp 35 which has been pushed out is transferred to.