A finisher that is capable of accurate sheet processing but is small in size, the finisher controlling a feeder and a sheet folding unit and executing 1) an operation in which the sheet is conveyed in the first direction, and after a downstream edge thereof relative to the first direction is detected by the detector during conveyance, the sheet is conveyed in the second direction, 2) an operation in which after such sheet edge is detected by the detector during conveyance in the second direction, the conveyance of the sheet in the second direction is continued for a prescribed amount based on a sheet length along a direction of conveyance, whereupon the sheet is stopped, and 3) an operation in which after the sheet is stopped, the sheet folding unit is operated to create a fold line at a prescribed position in the sheet.