When a size of a document is not specified, a control unit of an image reading device identifies, for image data generated as a reading result by a reading unit, a region of the document by detecting an edge of the document. When the size of the document is specified, the control unit searches the image data, at a position within a first range including a position away from an origin by a first distance, for a downstream end of the document from a downstream end of the image data toward upstream in a transport direction, and based on a position of the downstream end of the document identified by the searching, identifies as a region of the document a region corresponding to the size of the document specified.