To provide a device for binding a paper sheet bundle having a curling prevention mechanism in which, when a plurality of stacked sheets of paper are being conveyed, the lowermost sheet of the stacked paper sheets is prevented from curling up.

The device for binding a paper sheet bundle is provided for conveying and binding stacked 500 sheets of paper to form a bundle of 500 paper sheets, wherein 5 bundles, each bundle consisting of 100 sheets of paper, are stacked up to make the stack. The binding device includes: two conveyance bases 20a, 20b disposed separately on the upstream side and on the downstream side of conveyance where the stacked 500 paper sheets are conveyed; a band discharge groove 40 provided between these two conveyance bases; a plate spring 30 disposed at the end of the conveyance base on the upstream side of conveyance; a groove cover plate 10 for opening and closing the band discharge groove; and a pusher for pushing out the stacked 500 paper sheets. The device is configured such that: when the stacked 500 paper sheets pass over the band discharge groove, the tip end of the 500 paper sheets is lifted by the plate spring, and the groove cover plate covers the band discharge groove to allow the 500 paper sheets to pass over the groove; and, as the tip end of the 500 paper sheets moves in the conveyance direction, a supporting member which fixes the groove cover plate is pushed out in the conveyance direction to open the band discharge groove; and the 500 paper sheets are bound with a binding band.