Document image data is converted into luminance image data. Block image data is generated from the luminance image data. Start point block pixel data and end point block pixel data are set based on a change rate of luminance values of adjacent block pixel data. A luminance value of block pixel data between the start point block pixel data and the end point block pixel data is corrected based on a start point luminance value and an end point luminance value. The luminance value, of the block pixel data wherein each luminance value of an adjacent block pixel data cluster is less than an average value, is substituted by the average value. The luminance value of each pixel data is set based on the luminance value of the block pixel data. Post process luminance image data that includes each pixel data is reconverted into the document image data.