1. An image processing apparatus comprising: a data processing unit for processing data consisting of pixel values at coordinates in an original image to create data consisting of resultant pixel values at coordinates as data of a processed image; and a filtering unit for setting each of the coordinates in the processed image of the data created by the data processing unit as an attention coordinate to be processed, and defining as a filter window a region containing plural coordinates including the attention coordinate, and for performing a weighted-average filtering operation on data of the filter window, thereby converting the pixel value at the attention coordinate, wherein the filtering unit comprises: a filter coefficient calculating unit for using a first pixel value at a first coordinate in the original image, which first coordinate corresponds to the attention coordinate in the filter window, and a second pixel value at a second coordinate in the original image, which second coordinate corresponds to a certain coordinate in the filter window to calculate a filter coefficient at the certain coordinate on a coordinate to coordinate basis in the filter window; and a weighted-average filtering unit for using the filter coefficient of each coordinate in the filter window to apply the weighted-average filtering operation on data of the filter window.