1. A document reading apparatus, comprising: a 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, said image pickup device 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, said multi-value image data being digital data that corresponds to the image reading signal, 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 or the document in a sub-scanning direction; and an image data processing unit including a determination unit configured to identify a locally patent image pixel based on the multi-value image data by comparing a group of pixels formed by a target pixel and a plurality of pixels adjacent to the target pixel in the main scanning direction with a noise-line crossing pattern, and identifying said target pixel as a locally patent image pixel when said group matches the noise-line crossing pattern, a counter configured to count the number of lines on which the locally patent image pixels are continuously detected at the same pixel position as preceding lines and output a counter value, the counter beginning to count at a starting edge of sub-scanning during document reading, and a replacement unit configured to compare the counter value with a reference value based on a ratio of character height to character line width and to replace the multi-value image data of any locally patent image pixels detected by the determination unit and a predetermined range of pixels in the main scanning direction relative thereto, after the counter reaches the reference value, with multi-value image data having a value that levels the locally patent image pixels.