<p>In order to clarify the reason that the multi-feeding is harder to occur in an overlap-type sheet separation mechanism than in a pressing-type, we examined the behaviors of the paper sheet in the both sheet separation mechanisms. The apparatus that handle paper sheets, such as copy machines, printers, and ATMs, are widely used in these days. These apparatus are equipped with sheet separation mechanism to make paper sheet separate one by one. The sheet separation mechanisms are classified roughly into two groups, one is a pressing-type and the other is an overlap-type. The pressing-type separation mechanism is widely used in copy machines, printers, and facsimile, because it is simple and low cost, however, its reliability is relative low because a multi-feeding is easily to occur. On the other hand, the overlap-type separation mechanism is used in apparatus that high reliability is required such as ATMs although the mechanism is complex and high cost, because a multi-feeding is hard to occur in this mechanism. However, the reason why the multi-feeding is hard to occur in the overlap-type sheet separation mechanism has not been clarified. First, we introduced the model and formulation that can calculate the resistance force and feed force acting on the paper sheets in the overlap-type mechanism. And, based on the calculation results of the resistance force and friction force between the paper sheets, we examined the behavior of the paper sheet. As a result, we found that, in the overlap-type sheet separation mechanism, when the piled paper sheets move between the feed-roller and the gate roller, a slippage caused by the resistance force that is proportional to the number of the piled sheet occurs between the piled paper sheet, and this slippage is the factor that is superior to the pressing-type sheet separation mechanism.</p>