To provide a sheet punching unit that performs skew correction on the basis of a skew amount of a rear end of a sheet and opens punch holes when the sheet is a predetermined sheet.

A sheet punching unit comprises: a skew detector that includes first and second sensors disposed in a direction perpendicular to a transportation direction of a sheet and detects a skew amount of the sheet on the basis of a time difference between time when an end portion of the sheet in the transportation direction passes through the first and second sensors; a puncher that is provided downstream the skew detector and has punching blades for opening punch holes on the sheet; an orientation control mechanism for changing an angle of inclination of the puncher; and a controller that determines whether or not the sheet is a predetermined sheet, controls the angle of inclination of the puncher on the basis of the skew amount of a front edge and a rear end of the sheet detected by the skew detector when determining the sheet not to be the predetermined sheet and controls the angle of inclination of the puncher in response to the skew amount of the rear end of the sheet when determining the sheet to be the predetermined sheet.