A paper-sheet stacking apparatus comprising: a holding unit having a stacking base for holding a plurality of paper sheets in standing position and configured to stack the paper sheets, one laid on another in a direction of a plane; a pushing mechanism configured to push paper sheets transported, into the holding unit; a backup opposed to the pushing mechanism, able to move a predetermined distance in the direction the paper sheets are stacked and configured to hold any paper sheet stacked between the backup and the pushing mechanism and to move away from the pushing mechanism as paper sheets are stacked one after another; and a variable-force spring mechanism configured to bias the backup toward the pushing mechanism, to apply a first spring force to the backup while the backup is moving from the pushing mechanism by a first distance of the predetermined distance in a paper-sheet stacking direction in the initial stacking phase, and to apply a second spring force smaller than the first spring force to the backup while the backup is further moving by a second distance which is a remainder of the predetermined distance in the paper-sheet stacking direction in the middle stacking phase, characterised in that the stacking base has a flat support surface, guide ribs (27a, 27b, 27c) protruding from the support surface and configured to support a lower edge of the paper sheet, each extending straight from the pushing mechanism in the paper-sheet stacking direction, and of each guide rib, a part located with the first distance is made of a high-friction material, and a part located within the second distance is made of a low-friction material that offers less frictional resistance than the high-friction material, such that a coefficient of friction between the lower edge of the paper sheet and the low-friction material is smaller than a coefficient of friction between the lower edge of the paper sheet and the high-friction material. a holding unit having a stacking base for holding a plurality of paper sheets in standing position and configured to stack the paper sheets, one laid on another in a direction of a plane; a pushing mechanism configured to push paper sheets transported, into the holding unit; a backup opposed to the pushing mechanism, able to move a predetermined distance in the direction the paper sheets are stacked and configured to hold any paper sheet stacked between the backup and the pushing mechanism and to move away from the pushing mechanism as paper sheets are stacked one after another; and a variable-force spring mechanism configured to bias the backup toward the pushing mechanism, to apply a first spring force to the backup while the backup is moving from the pushing mechanism by a first distance of the predetermined distance in a paper-sheet stacking direction in the initial stacking phase, and to apply a second spring force smaller than the first spring force to the backup while the backup is further moving by a second distance which is a remainder of the predetermined distance in the paper-sheet stacking direction in the middle stacking phase, characterised in that the stacking base has a flat support surface, guide ribs (27a, 27b, 27c) protruding from the support surface and configured to support a lower edge of the paper sheet, each extending straight from the pushing mechanism in the paper-sheet stacking direction, and of each guide rib, a part located with the first distance is made of a high-friction material, and a part located within the second distance is made of a low-friction material that offers less frictional resistance than the high-friction material, such that a coefficient of friction between the lower edge of the paper sheet and the low-friction material is smaller than a coefficient of friction between the lower edge of the paper sheet and the high-friction material.