To provide a sheet binding device capable of performing binding processing according to the moisture content of a sheet.

A sheet binding device 30, which binds a sheet bundle PB by forming concave and convex patterns of a thickness direction in a sheet bundle including a plurality of sheets, comprises: a pressing device 47 including a pair of lower tooth member 31 and an upper tooth member 32 contacted with/separated from each other and configured to hold the sheet bundle to form concave and convex patterns and capable of changing a pressing force applied to the sheet bundle via the tooth members 31 and 32; moisture information acquisition means for acquiring moisture information regarding the moisture of the sheet bundle; and a CPU (control means) 51 for changing the pressing force applied to the sheet bundle by the pressing device 47 based on the moisture information acquired by the moisture information acquisition means. The CPU 51 changes the pressing force corresponding to the change of the moisture content of the sheet based on the moisture information.