An object of the present invention is to provide an image processing method for reducing noises without causing a side effect and an X-ray CT system that performs the image processing. An input image is wavelet-transformed using a gradient form. Resultant wavelet vectors are classified into categories of strongly edged objects, weakly edged objects, radial lines, and noises. The wavelet vectors classified into the category of strongly edged object are left intact. The wavelet vectors classified into the category of weakly edged objects have the directions thereof filtered. The wavelet vectors classified into the categories of radial lines and noises are smoothed. The resultant wavelet vectors are inverse-wavelet-transformed in order to produce an output image.