Sheet processing apparatus and sheet processing method

An image forming apparatus comprising:

• an image forming unit having an operation panel and a printer unit for forming an image on a sheet;
• a conveying motor for conveying the sheet outputted from the image forming unit along a conveying path;
• a hole punching section arranged, in order to perform punching processing to a sheet conveyed thereto, orthogonally to the conveying path of the sheet;
• a moving mechanism configured to move the hole punching section along a direction orthogonal to the conveying path and move the hole punching section in a direction toward a retreat position and a punching processing position;
• a rotating mechanism which tilts the hole punching section in the conveying direction of the sheet;
• a detection unit arranged upstream from the hole punching section and configured to move by being interlocked with the hole punching section and detect a lateral edge of the sheet conveyed thereto;
• a control unit configured to control the moving mechanism in accordance with a result of detection by the detection unit and vary the position of the hole punching section, thus controlling the punching processing position with respect to the sheet;
• and
• a skew correction unit configured to control the rotating mechanism in accordance with a quantity of skew of the conveyed sheet, and carry out skew correction by changing tilt angle of the hole punching section;
• wherein the control unit causes the hole punching section to be moved in the direction to the retreat position along the orthogonal to the conveying path with the conveying of the sheet, causes the hole punching section to retreat by a prescribed quantity with reference to the result of detection by the detection unit, then causes the hole punching section to be moved in the opposite direction, and controls movement to the punching processing position in accordance with the result of detection by the detection unit.