An image reading apparatus includes a reading processing unit that acquires image data including an image of a document placed on a document placement unit. A first detection processing unit detects a first line image whose width or density exceeds a predetermined first threshold value from the image data. A second detection processing unit detects a rectangular area that is defined by the first line image, included in the image data, and satisfies a predetermined detection condition. A third detection processing unit detects a second line image whose width or density exceeds a second threshold value smaller than the first threshold value from the rectangular area. A fourth detection processing unit detects, as document areas of the respective documents, the rectangular area not divided by the second line image and divided areas included in the rectangular area divided by the second line image.