An automatic document feeder comprising: a single scan path having a document admission passage to admit a document; a document scanning part to scan the document in single and duplex scanning modes; a document discharge passage to discharge the scanned document; a duplex scan path having a first document reversing and waiting passage diverged separately from the document discharge passage on the single scan path at a portion downstream of the document scanning part to reverse the document, one surface of which is scanned and admit the reversed document again to the document scanning part; a document re-reversing path having a second document reversing and waiting passage diverged separately from the document discharge passage on the single scan path downstream of the document scanning part to reverse the document, both surfaces of which are scanned, again and send the re-reversed document to the document discharge passage; and a document guide unit formed at a diverging point between the first document reversing and waiting passage and the second document reversing and waiting passage to the single scan path to guide the scanned document to a corresponding passage, characterised in that the document guide unit comprises: upper and lower fixed guides (141,142) disposed above and below the diverging point, respectively; a single movable guide disposed to be rotatable about an axis between the upper and lower fixed guides (141,142), and having an upper guide part, a lower guide part, and a passage formed between the upper guide part and the lower guide part; and a rotation driving unit to rotate the movable guide in a position corresponding to the scanning mode. a single scan path having a document admission passage to admit a document; a document scanning part to scan the document in single and duplex scanning modes; a document discharge passage to discharge the scanned document; a duplex scan path having a first document reversing and waiting passage diverged separately from the document discharge passage on the single scan path at a portion downstream of the document scanning part to reverse the document, one surface of which is scanned and admit the reversed document again to the document scanning part; a document re-reversing path having a second document reversing and waiting passage diverged separately from the document discharge passage on the single scan path downstream of the document scanning part to reverse the document, both surfaces of which are scanned, again and send the re-reversed document to the document discharge passage; and a document guide unit formed at a diverging point between the first document reversing and waiting passage and the second document reversing and waiting passage to the single scan path to guide the scanned document to a corresponding passage, characterised in that the document guide unit comprises: upper and lower fixed guides (141,142) disposed above and below the diverging point, respectively; a single movable guide disposed to be rotatable about an axis between the upper and lower fixed guides (141,142), and having an upper guide part, a lower guide part, and a passage formed between the upper guide part and the lower guide part; and a rotation driving unit to rotate the movable guide in a position corresponding to the scanning mode. upper and lower fixed guides (141,142) disposed above and below the diverging point, respectively; a single movable guide disposed to be rotatable about an axis between the upper and lower fixed guides (141,142), and having an upper guide part, a lower guide part, and a passage formed between the upper guide part and the lower guide part; and a rotation driving unit to rotate the movable guide in a position corresponding to the scanning mode.