A post-processing device includes: a tray; a sheet output section; a stapler provided movably in a direction perpendicular to a direction of discharge of a stack of paper sheets and configured to apply staples to a plurality of locations on the stack of paper sheets loaded on the tray; a sheet stack shifting section configured to shift the stack of paper sheets loaded on the tray in the direction perpendicular to the direction of discharge of the stack; and an operation control section configured to allow the stapler to apply a staple to a first location on the stack of paper sheets and then allow the stapler to move to a position corresponding to a location to be stapled next while allowing the sheet stack shifting section to shift the stack in a direction opposite to a direction of the movement of the stapler.