1. An image processing method in which the image of a document is extracted from an image including the image of the document on a document positioning table, acquired by a line sensor scanning the document placed on the document positioning table while illuminating the document from the upstream side or the downstream side of the scanning, the method comprising: extracting the edge of the image on the document positioning table to set a provisional document area based on the edge; detecting a bright area where light incident on the line sensor is excessive, at the illuminated edge on the upstream side or the downstream side in the scanning direction, from the image of the provisional area; and determining the area of the document by removing the area where light is excessive from the provisional area.