1. An auto document feeding device comprising: a first conveying path configured to reach from a document placing unit to a first image reading unit, the first conveying path including a first reversing unit formed in an arc shape, the first conveying path being an OUT path including the first reversing unit, a second conveying path configured to reach from the document placing unit to the first image reading unit and have a path different from the first conveying path, the second conveying path having a second reversing unit formed in an arc shape, the second conveying path being an IN path including the second reversing unit having a radius smaller than a radius of the first reversing unit; a gate unit configured to direct original documents, which are sent from the document placing unit, to either the first conveying path or the second conveying path, the gate directing the original document to the OUT path when the original document has a thickness equal to or larger than a fixed thickness; a document conveying unit includes a first conveyance mode which conveying the original documents with overlap a trailing edge of a preceding original document and a leading edge of a following original document, when the gate unit alternately directs the original documents from the document placing unit to the first conveying path or the second conveying path; and a separating unit configured to separate the overlap of the trailing edge of the preceding original document and the leading edge of the following original document before the leading edge of the following original document reaches the first image reading unit.
2. The device according to claim 1 , wherein the separating unit includes a first roller and a second roller that rotate following each other, and the first roller and the second roller switch driving rotation for conveying the original document to the first image reading unit and the following rotation.