A sheet processing apparatus, comprising: a sheet introduction port for feeding a sheet to inside from outside of the sheet processing apparatus; a sheet exit port for ejecting the sheet from inside to outside of the sheet processing apparatus; a sheet carrying part having a sheet carrying passage that communicates the sheet introduction port and the sheet exit port; a fold processing part disposed in a middle of the sheet carrying passage, for arbitrarily folding the sheet; a sheet stacking part that stacks the sheet ejected from the sheet exit port; and a detector that detects whether a height of a bundle of sheets stacked on the sheet stacking part reaches a prescribed height and outputs an OFF signal for turning off a drive of the sheet carrying part, wherein the detector performs detection at a first detection level for detecting the prescribed height of the bundle of sheets folded when the fold processing part is set in a drive state, and at a second detection level for detecting the prescribed height of the bundle of sheets not folded when the fold processing part is in a stop state, with the first detection level and second detection level differentiated from each other.