An image reading apparatus includes a document tray, and an automatic document feeding unit disposed at an upper side of the document tray to transport documents as to pass through the upper side of the document tray. The automatic document feeding unit includes a body; a document supplying tray having a first width corresponding to a width of a first document to be scanned and at which the first document to be scanned is loaded; and an auxiliary guide having a second width that is smaller than the first width to correspond to a second document having a width smaller than the width of the first document and is installed at the document supplying tray. The auxiliary guide includes a guide portion to support both widthwise sides of the document to be scanned while entered toward an upper side of the mounting unit.