PURPOSE: To prevent a rod in a jack from being damaged even when jamming occurs in a stacker, so as to eliminate the necessity of replacement of jacks.

CONSTITUTION: A sheet supporting mechanism is composed of an upper sheet supporting grid 14 and a lower sheet supporting grid 9 which is composed of rods 15, 16 of jacks 10, 11, 18 arranged between a front wall 7 and a rear wall 8 of a stacker 8. The upper grid 14 is composed of a series of jacks 17, 18 having rods 15, 16 which are not permanently but elastically deformable. The rods 15, 16 are fitted in pistons which are not permanently but elastically deformable pistons. The rods 15, 16 which are not permanently but elastically deformable are guided by the circumferential surfaces of the pistons which slide in bearings and in cylinders in the jacks 17, 18 during movement.