To provide a sheet processing device capable of easily determining a sheet bundle in which a staple position and a folding position deviate.

In a sheet bundle in which a folding position to be its leading edge and a staple position deviate (Fig. 8(a)), the staple position is on the upstream side in a conveyance direction of the folding position. Thus, a time difference Δt1 occurs between a time T2 elapsed since a folding conveyance motor M4 started to convey the sheet bundle until a sheet bundle detection sensor S8 detects the leading edge of the sheet bundle and respective times T11 and T12 elapsed until first and second staple detection sensors S9 and S10 detect a staple. If time differences between a detection timing of the sheet bundle detection sensor S8 and detection timings of the first and second staple detection sensors S9 and S10 are not less than a predetermined time K while an error in staple processing is added thereto, it is determined that the folding position and the staple position deviate.