To provide a sheet processing device capable of controlling the number of folding by detecting the sheet fiber orientation of the sheets and adding a result of detection to the sheet information about the sheets forming a bundle of sheets, and improved in folding grade of the bundle of sheets, and shortened in the binding time, and to provide an image forming device with the same.

Sheet fiber orientation of the sheets in conveyance is detected by a detecting device using a terahertz wave, and folding condition such as single folding and double folding is changed based on a folding mode selecting table predetermined according to the sheet fiber orientation, the number and basis weight of the sheets. Folding grade of the bundle of sheets is thereby improved, and the binding time can be shortened.