The present invention is to provide a sheet bundle binding device capable of easily removing some sheets from a sheet bundle that has been subjected to staple-free binding. A corner Sc of a first sheet bundle which is accumulated on a processing tray is subjected to proper binding using a staple needle by a staple binding unit. Then, a corner of a second sheet bundle obtained by accumulating additional sheets on the first sheet bundle is pressed and deformed between crimping toothed parts of a staple-free binding unit to temporarily bind the second sheet bundle. Thus, the additional sheets can easily be removed from the second sheet bundle.