A sheet processing apparatus is described which is used to process a sheet transferred from an image forming apparatus such as a copying machine. A sheet processing apparatus is provided with a punching unit which cuts off part of a sheet conveyed along a conveying route, and a humidifying unit located on this conveying route in the upstream side of the punching unit and configured to humidify a predetermined area which is selected from the entire area of the sheet and includes punch areas to be cut off from the conveyed sheet by the punching unit. The cutting wastes can thereby be electrostatically discharged without leaving the sheet excessively wet.