1. An image processing device for producing a binary image from a scanned multi-valued gray image, comprising: an image scanning unit for scanning an image as said multi-valued image; an image memory for storing said multi-valued image captured by said image scanning unit; a sample area designating unit for designating a sample area in said scanned multi-valued image stored in said image memory; a binarization unit for implementing a filtering process on said scanned multi-valued image over which said sample area is designated, to produce the compensated image data and for binarizing said compensated image data; a correlation operation unit for acquiring each of the correlations between the binary image data acquired at said binarization unit and said scanned multi-valued image data; and an optimal binarization information selecting unit for selecting the optimal parameter value or binary image data based on said correlations; wherein said binarization unit includes a parameter value setting section for setting multiple parameter values which are calculated by adding the variable additaments to the coefficient for a target pixel in the filter, a filtering process section for creating a matrix that contains said target pixel in the center and its neighboring pixels and for implementing a filtering process on said scanned multi-valued image data using said filter for which said parameter values are set, to produce the compensated image data corresponding to said parameter values, and a binarization process section for binarizing each of said compensated image data [values] based on a predetermined binary threshold.