To minimize a stroke of an air cylinder when releasing a metal plate by automatically adjusting the position of the air cylinder itself in accordance with the thickness of the metal plate.

After clamping the metal plate 258 with an upper feed roller 248 and a lower feed roller 250 by lowering the air cylinders 230, 232 by rotationally driving a geared motor 224 and the eccentric shaft 222 of a clevis pin until the upper feed roller 248 is struck to the metal plate 258 when the metal plate 258 is conveyed between the upper feed roller 248 and the lower feed roller 250, releasing operation for releasing the metal plate 258 is performed by elevating pistons 230b, 232b by a prescribed stroke equivalent to the amount C of release.