To provide a sheet processing device capable of suppressing occurrence of a loading failure in the first sheet bundle.

A processing section 15 applies a folding process to a paper bundle P. A tray 30 includes a tray body including a tray surface for mounting a plurality of paper bundles P output from the processing section 15, and a transport section including a belt extending in an x-axis direction on the tray surface for transporting the paper bundle P toward a positive direction side in the x-axis direction by drive of the belt. A control section 26 causes the processing section 15 to output the first paper bundle P for the tray body in a state where the belt is driven toward the positive direction side in the x-axis direction.