An upstream-side unit of a reading unit of an automatic document feeder has protruding portions that protrude toward a surface that forms a document feeding path in a mechanism body unit on both sides of a document in the width direction outside a feeding portion that forms the document feeding path. Positioning of the upstream-side unit provided so as to be rotatable with respect to the mechanism body unit is performed by making protruding portions come in contact with a guide surface of the mechanism body unit, the guide surface forming the document feeding path. By adopting this configuration, because the protruding portions provided on both sides of the document in the width direction outside the document feeding path are used to perform positioning, it is possible to increase the accuracy of clearance for allowing a document to pass through the document feeding path.