1. An image reading apparatus comprising: an image processor configured to act as: an image data obtaining unit that obtains, as image data, a read signal read from a document that passes over a read area; a foreign material position information obtaining unit that obtains, as foreign material position information, a read signal read from the read area in which the document is not present; and a background removing unit; and a separation processor that separates the image data into a plurality of objects including a background object corresponding to a background of the document, detects, by using the foreign material position information, a streak image portion that is caused by foreign material present in the read area and that is included in the image data, and performs a process for assuming a pixel of the streak image portion to belong to the background object if the pixel is present in the background object and has a pixel value equal to or smaller than a threshold determined in accordance with a representative value of the background, wherein when separating the image data into the plurality of objects, the separation processor separates a pixel of the streak image portion having a pixel value equal to or smaller than the representative value of the background as a pixel belonging to the background object, and assumes and separates a pixel of the streak image portion having a pixel value larger than the representative value of the background as a pixel belonging to a line object, and the background removing unit converts a pixel value of a pixel that belongs to the background object separated by the separation processor by using a relationship between an input value and an output value and by using the pixel value as the input value, and converts a pixel value of a pixel that is present in the background object and that is assumed, by the separation processor, to belong to the line object into the representative value of the background.