Document image reading device

A document-image reading device comprising a document image reading means for reading image data from a document at a specified time-interval based on a reference clock by moving said document image reading means or the document in a feed direction, wherein said document-image reading means is provided with reference-chart reading means for reading a reference chart representing alternations of a low-density or high-density divisions with a width equal to a least readable pitch and a high-density or low-density division with a width equal to an integer-multiple of the least readable pitch along the feed direction and image-data processing means for separating the high-density divisions and low-density divisions from the read image-data, determining a correction factor by which image data for all low-density or high-density divisions or respective low-density or high-density divisions can be corrected to have a constant value, multiplying the image data by the correction factor, processing the image data read from the reference chart by using a comb filter, performing integral calculations on the filtered data, extracting continuous detection of the substantially same integrated value by the integer times and determining an integrated value at the extracted position by subtracting the integer-fold value of the least-readable pitch at the upstream side in the feed direction from the extracted integrated value.