To surely detect size by every document and to feed the document to a prescribed read position according to the size in a document feeder even when the document is erroneously set or the different-sized documents are set by coexisting.

This document feeder 2 is constituted so that the documents placed on a tray 4 are fed to the read position corresponding to the length of the document out of the plural read positions where the image of the document is read. Then, length detection means 101c and 101d detecting the length of the document are arranged at the middle part of a feeding path for feeding the document. Based on the length information of the document obtained by the length detection means, the read position is decided.