The invention discloses an auto document feeding device and a document feeding method, wherein the lengths of an OUT path and an IN path are made longer than the length of a document of a main size in a traveling direction. The document of the main size stands by for conveyance in a direction of a READ document glass in a state where the entire document is contained in the OUT path or the IN path. The entrance speed of the document into the OUT path or the IN path is equal to the read speed of a scanner. The leading edge of a subsequent document is overlapped on the trailing edge of a preceding document, and the document is conveyed to the READ document glass.