To obtain a structure by which accuracy in punching a sheet can be improved without increasing the size of the device or complicating the same.

Skewing of a sheet is corrected by allowing the sheet to abut on an abutment member 64. Punching is performed by a punch unit 60, serving as punching means, at a predetermined width-directional position with respect to the sheet. On this occasion, the sheet is conveyed by a shift roller unit 110 serving as conveying means, and the position of the sheet in the width direction is detected at two or more positions in a conveying direction by an end detecting sensor S3 serving as width-directional position detecting means, whereby the amount of deviation of the sheet from the predetermined width-directional position after abutment of the sheet on the abutment member 64 is computed. Next, the sheet or the punch unit 60 is moved based on the amount of deviation, and then the sheet is punched.