An image reading device includes: a supplying tray on which a document is placed; a document conveyer that successively conveys the document placed on the supplying tray toward a reading position; a plurality of detection sensors that are disposed in a plurality of positions in a width direction intersecting a conveyance direction of the document, and detect a front edge of the document in the conveyance direction when the document is placed on the supplying tray; and a document reading portion that reads an image on the document passing the reading position, and generates an image signal.