1. An image reading apparatus for reading a book document mounted on a document stand as opened upward, comprising: an image reading device for reading the document mounted on the document stand and producing an image as an output; a memory device for storing the output image; a longitudinal edge detecting device for detecting a longitudinal edge in the stored image; a pattern detecting device for detecting a prescribed pattern formed by the detected longitudinal edge; and an image reading area deciding device for deciding an area to be read by said image reading device based on the detected pattern.