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 Sb1 accumulated on a processing tray 24 is pressed and deformed between crimping toothed parts 54 and 55 of a staple-free binding unit 51 to bind the first sheet bundle Sb1 with a first binding force. Then, a corner of a second sheet bundle Sb2 obtained by accumulating additional sheets Sh2 on the first sheet bundle Sb1 is pressed and deformed between the crimping toothed parts 54 and 55 of the staple-free binding unit 51 to bind the second sheet bundle Sb2 with a second binding force smaller than the first binding force. Thus, the additional sheets can easily be removed from the second sheet bundle Sb2.