The invention provides an auto document feeding device, image reading device and auto document feeding method. An auto document feeding device causes an original document passing through an OUT path and an original document passing through an IN path to flow together with overlap a trailing edge of a preceding original document and a leading edge of a following original document. Before the original documents reach to a READ document glass, the auto document feeding device comprises a separating unit configured to separate an overlapping portion of the original documents. The auto document feeding device reduces a distance between the preceding original document and the following original document passing on the READ document glass to nearly 0 mm.