When the rear end of a sheet bundle S is brought to reach the clamping portion between a rocking roller and a rear end aligning wall by conveyance means, the individual timings, at which the rocking roller and the rear end aligning wall move to the escape position, are controlled to drop said sheet bundle S onto a stack tray, and then the rear end aligning wall is moved from the escape position to the support position thereby to register the rear end of the sheet bundle S dropped on the stack tray.