To correct seat curl while suppressing decline in throughput.

This sheet processing device has a pair of upstream paper delivery trays and a pair of downstream paper delivery trays carrying sheets, a staple processing tray on which carried sheets are loaded, and a knurl belt (curl correction means) correcting curl of the sheets on the staple processing tray. The sheet processing device is further provided with a curl amount acquiring means 50 acquiring curl amount of carried sheets; a comparing means 51 the curl amount acquired by the curl amount acquiring means 50 with reference curl amount; and an MPU (control means) 41 switching a first mode which performs curl correction by a curl correction means 50 and a second mode which does not perform curl correction, based on the comparison result by the comparing means 51.