A sheet processing apparatus, by which a failure in control does not occur even if cutting wastes are conveyed by a conveying unit and an image forming apparatus having the sheet processing apparatus are provided. The sheet processing apparatus has a cutter unit for cutting a sheet, a dust box for accommodating the cutting wastes, a conveying belt pair which conveys the cut sheet, sensors for detecting the sheet conveyed by the conveying belt pair, and a controller for controlling conveyance of the sheet based on the signals detected by the sensors, wherein when the length of the sheet detected by the sensor is equal to or less than a predetermined amount, the controller does not determine the sheet as a sheet.