1. An image correcting apparatus for correcting a scanned image of a book document read by image reading means through a contact glass while said book document is in contact with said contact glass, said apparatus comprising: divergent angle detecting means for detecting a divergent angle of said scanned image using a linear portion included in said scanned image of said book document; and divergent distortion correcting means for correcting said scanned image for a divergent distortion based on said divergent distortion angle detected by said divergent distortion angle detecting means and subsequently correcting said scanned image for a shape distortion in a sub-scanning direction.