1. A document reading apparatus, comprising: a document scanner and an image data processing unit; said document scanner including an image pickup device including a photo-electric conversion element array constituted by a plurality of photo-electric conversion elements arranged in a line in a main scanning direction, and configured to repeatedly output an image reading signal provided by said photo-electric conversion element array for every main scanning line; a conversion unit configured to convert the image reading signal into multi-value image data of a pixel on the main scanning line, the multi-value image data being digital data that express the image reading signal by multiple values; an optical system configured to project an image of a document to the image pickup device; and a sub-scanning unit configured to drive one of the optical system and the document in relation to the other in a sub-scanning direction; and said image data processing unit being configured to determine whether each pixel on the main scanning line is a locally patent image pixel based on the multi-value image data, where the locally patent image pixel is a pixel, a patent image level of which is locally high; to count the number of lines on which the locally patent image pixels are continuously detected at the same pixel position as preceding lines as sub-scanning proceeds; and to replace the multi-value image data of locally patent image pixels beyond a position corresponding to the reference value with multi-value image data having a value that levels the locally patent image pixels.