Image data of a plurality of lines in a document are read in the main scan direction and/or in the subscan direction, and the document size is determined on the basis of the image data of the plurality of lines. Thus, the erroneous detection of document size can be avoided. The document size is determined for example as an average of the document sizes detected for lines except the smallest and largest document sizes. When the same anomaly is observed twice, new lines are used to determine the document size. Further, an image reading apparatus reads image data of a line in a document in the main scan direction and in the subscan direction with a linear image sensor. When the document size is detected on the basis of the image data of a plurality of lines in the main scan direction, the document is scanned in the subscan direction at a fast read rate, while the document size is detected on the basis of the image data of a plurality of lines in the subscan direction, the document is scanned in the subscan direction at a low read rate.