A tone step detector detects whether the tone of n-bit input image data varies gradually, and generates a tone processing control signal indicating that a tone step is contained. A bit shifter bit-shifts the n-bit input image data to the left by α bits, to generate (n+α)-bit image data. A tone processor smoothes the image data of the (n+α)-bit image data generated by the bit shifter, in the region where the tone processing control signal indicates that a tone step is contained, to generate (n+α)-bit image data having been smoothed in the above-mentioned region. Because of the selective smoothing, degradation in the image quality due to quantization is reduced even when an analog image signal whose tone varies gradually is input.