1. An image processing method for filtering image data, which is constituted with a plurality of pixels, at each pixel position by using pixel values indicated by pixels present within a predetermined range containing a target pixel, comprising: determining two arguments that are a first argument defined by a spatial distance from the target pixel and a second argument defined by a difference in signal intensity relative to the target pixel, in correspondence to each of the pixels present within the predetermined range; obtaining a weighting coefficient to be used when filtering each of the pixels present within the predetermined range, based upon the first argument and the second argument; determining the weighting coefficient by ensuring that w(A1,B1)/w(A1,B2)≠w(A2,B1)/w(A2,B2) is true with A1 and A2 representing different values taken for the first argument, B1 and B2 representing different values taken for the second argument, and w(An, Bn) representing the weighting coefficient being obtained; and filtering the image data by using a filter coefficient corresponding to the weighting coefficient having been obtained.