A document scanner (111) includes a document tray (2) for supporting a plurality of documents (11) to be read, a reading section (61) for reading contents of the documents, and a document feeder (4) for feeding the documents one by one to the reading section from the document tray. A code decoder (63) is provided in the read section for reading a code (12) provided at a leading edge of each of the documents. A detecting unit (64) is connected to the code decoder for detecting multiple feeding when the code decoder is unable to read the code which is covered by the preceding document.