The present invention relates to a paper sheet folding device and more particularly to such a device adapted to fold a packet of several paper sheets at one time. 
A typical construction of such a paper sheet folding device of the prior art is disclosed, for example, by Japanese Utility Model Registration No.  1592186 (Utility Model Publication No.  59-29007).  This device comprises a paper feeder mechanism, a folding plate provided with an adjustably movable stopper and a pair of folding rollers.  The paper sheet is fed by the paper feeder mechanism to the folding plate in such manner that the forward end thereof comes in contact with the stopper of said folding plate to be flexed, then the flexed portion of the paper sheet is grasped between the folding rollers to fold the paper sheet. 
The device of the prior art as has been described above cannot be used to fold a packet of paper sheets because its paper feeder mechanism is not adapted to feed the packet of several paper sheets at once and also because a folding deviation tends to occur between each pair of adjacent paper sheets within each packet when the latter is folded at once.  This deviation occurs since a location along which each paper sheet is folded depends upon factors such as the degree of paper sheet flexure and the timing of the paper sheet being grasped by the folding rollers.  Thus, this device is certainly suitable for continuously handling a plurality of paper sheets having the same size one by one, but inconvenient when it is desired to fold a packet of paper sheets at once into a predetermined size, as when several letter sheets are folded to be inserted into each envelope.  Accordingly, it has been a usual practice to manually fold each packet of letter sheets for insertion into each envelope.  However, such a manual operation is extremely troublesome and inconvenient in that the size of each sheet after being folded cannot be uniform within each packet.