An original image is transformed into a multi-resolution space and is thereby decomposed into images, each of which is of one of a plurality of different frequency bands. An image of the lowest frequency band, which is lowest among the plurality of the different frequency bands, is processed with the formulag.sub.L '=g.sub.L +f.sub.1 (g.sub.L)=f.sub.2 (g.sub.L)wherein f.sub.1 (g.sub.L) represents a function, the value of which decreases monotonically as the signal value g.sub.L of the image of the lowest frequency band increases. A processed image of the lowest frequency band is obtained from the processing. An inverse multi-resolution transform is then carried out on the processed image of the lowest frequency band and the images of the other frequency bands, and a processed image is thereby obtained.