SHEET PROCESSING DEVICE AND SHEET PROCESSING METHOD

PROBLEM TO BE SOLVED: To perform square-back processing without generating a trace of a middle fold in the spine of a booklet.SOLUTION: A control section perform control so as to include two modes, that is, a first folding mode for sandwiching with plural rollers a sheet in the state where a folding knife exists at a portion becoming a valley of a fold and then feeding out the sheet along with the folding knife by rotating the plural rollers, and a second folding mode for extracting the folding knife sandwiched by the sheet in the state where the plural rollers are stationary while positioning and pressing both ends of the sheet after sandwiching the sheet in the plural rollers and then feeding out the sheet by rotating the plural rollers.