A post-processing apparatus having a function of aligning and sequentially stacking paper output from an image forming apparatus is provided to align paper sheets output from the image forming apparatus in order and sequentially stack the paper sheets without using an additional device or means. A post-processing apparatus for an image forming apparatus includes an entrance guide, a stacking guide, a stopper bar assembly, and a snap roller assembly. The entrance guide has lower and upper entrance guide plates(10,20) arranged in parallel with each other and guides paper to enter in a predetermined direction. The stacking guide includes a lower stacking guide plate(50) located at an angle to the entrance direction and an upper stacking guide plate(60) forming a conic accommodating space with the lower stacking guide plate and is located under the entrance guide to stack paper sent from the entrance guide. The stopper bar assembly includes a vertically moving frame(65) and at least one stopper bar(70) fixed to the vertically moving frame and sets a lower limit of stacked paper. The snap roller assembly includes a snap roller axis(85) perpendicular to the paper entrance direction and at least one snap roller(80) which is fixed to the snap roller axis and has a plurality of friction protrusions and passes the back end of paper stopped around the snap roller to the lower stacking guide plate.
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