An automatic document feeder for an image forming apparatus, such as a scanner, is provided. The automatic document feeder comprises a document-loading means, a document-receiving means, a sheet path, a sheet-transporting mechanism, a scan window, a first sensor means and a second sensor means. The first sensor means is disposed at the entrance of the sheet path and the second sensor means is disposed at a predetermined position along the sheet path. The second sensor means includes a sensor device and a rotatable shaft attached thereto. When the sheet-transporting mechanism transports at least one sheet along the sheet path through the second sensor means, the rotatable shaft is rotated at a predetermined angle.