1. An image reading apparatus comprising: a document table; a read unit, which is movable along the document table, and which reads a document placed on the document table so as to generate image data representing a read image of the document; a partial prescanning unit, which conveys the read unit in a partial area of a readable area of the document table that can be read through the read unit, and which controls the read unit to execute read operation in the partial area; and a processor comprising: a corner estimation unit, which converts the image data of the read result of the read unit provided by the operation of the partial prescanning unit into edge image data representing an edge image, and which estimates position coordinates on the document table corresponding to two of four corners of the document on the assumption that the document placed on the document table is quadrangular based on the edge image data; and a document estimation unit that estimates the state of the document placed on the document table based on the estimation result of the corner estimation unit.