To increase the operability and document feeding quality by properly judging the use of switch back function.

A sheet thickness detecting part 17 is provided between a pair of pull-out rollers 15a and 15b and a pair of relay rollers 16a and 16b. Also the sheet thickness of a document to be fed is detected and, when the document comprises thin sheets, a feed belt 21 is not switched back but the document is set on a contact glass 3 at a reference position by merely rotating a feed belt 21 in normal direction. When the document comprises those sheets thicker than plain paper sheets, the feed belt 21 is switched back so as to brought the rear end of the document in contact with the side surface of a document scale 4 and to set it securely at the reference position.