Binding device and image forming system

A binding device comprising:

• a first pushing member that includes a plurality of protrusions along one direction and that pushes a recording-material bundle as a result of pushing the plurality of protrusions against one of surfaces of the recording-material bundle, the plurality of protrusions disposed side by side in a direction that intersects the one direction;
• and
• a second pushing member that includes a plurality of protrusions along the one direction and that pushes the recording-material bundle as a result of pushing the plurality of protrusions against the other surface of the recording-material bundle, the plurality of protrusions disposed side by side in the direction that intersects the one direction,
• wherein, in at least one end portion in the one direction of each protrusion of at least one of the first pushing member and the second pushing member, a hollow is disposed in a top portion of each protrusion.