For the purpose of enhancing a difference between pixels relating to a substantial structure in an image and other pixels, with each pixel constituting an image defined as a pixel of interest, the variance of pixel values is determined in a local region to which the pixel of interest belongs (508); and the pixel value of the pixel of interest is maintained when the variance of pixel values is significantly larger than the variance of noise, otherwise the pixel value of the pixel of interest is suppressed (510, 512).