In an automatic document feeder for use in an image forming device such as copy machine and facsimile machine: a document shunting path for temporarily shunting a document is provided at an upstream side of a document reading portion; in a different-size mixed mode where different sizes of documents are placed in a mixed manner, the leading edge and rear edge of the document are detected by a sensor during conveyance of the document in the direction toward the document reading portion, so that the length of the document is detected; thereafter the document is conveyed in the reverse direction so that the rear edge of the document is shunted into the document shunting path; and thereafter the document is again conveyed in the forward direction to conduct reading. Thus, it becomes unnecessary to provide a longer conveyance path for detecting the length of the documents, so that the size of the feeder itself can be reduced.