Unsharp mask image signals having different frequency response characteristics are formed from an original image signal, and band-limited image signals are formed from the unsharp mask image signals. First and second converting processes are carried out on the band-limited image signals, and first converted image signals and second converted image signals are thereby formed. The first converted image signals are integrated, and a signal concerning high frequency components is thereby obtained. Also, the second converted image signals are integrated, the thus obtained integration signal is subtracted from the original image signal, and a difference signal is thereby formed. The difference signal is converted, and a signal concerning low frequency components is thereby obtained. The signal concerning high frequency components and the signal concerning low frequency components are added to the original image signal. Thus the high frequency components of the original image signal are emphasized, and the dynamic range of the original image signal is compressed.