In the invention, for a first sheet, regardless of the sheet size (width), a lateral registration detector is moved in a direction towards an edge face of the sheet from a home position to detect the edge face of the sheet. With lateral deviation in the sheet position corrected, punching is performed by a puncher. For the second and subsequent sheets, the lateral registration detector is moved in advance to near the edge face of the sheet with reference to the detected position of the sheet edge of the first sheet, and the edge face is detected at a given timing. With lateral deviation in the sheet position corrected, punching is performed by the puncher.