An image reading device includes a document stacking tray on which documents are stacked, a feed roller for picking up, one sheet at a time, the documents that are stacked on the document stacking tray, a curved transporting path through which the document is transported, and a CCD reading unit and a CIS reading unit for reading images from the document. The document stacking tray includes a electric tray for raising or lowering the document within a range of a height of the curved transporting path, so as to feed the document to the feed roller. With such an arrangement, it is possible to provide an image reading device and an image forming apparatus that are capable of separating and transporting a large number of documents and efficiently reading images from the documents even when the documents are thick.