This invention can suppress overshoot/undershoot, and can maintain the improvement effect of the leading edge of an edge portion. For this purpose, a second derivative signal generator (1) generates a second derivative signal (s5) on the basis of time-serially input three successive pixels. A second derivative signal suppression unit (2) generates a signal (s8) obtained by suppressing the second derivative signal, and a signal (s9) indicating the sign of the second derivative signal. The absolute values of the differences between the pixel of interest, and its neighboring pixels are computed, and a smaller one of the absolute values is selected as a signal (s14) by a minimum value selection circuit (18). By multiplying the obtained signals (s8, s9, s14) by a multiplier (19), an edge-emphasizing signal (s15) is generated. The edge-emphasizing signal (s15) is added to the signal of the pixel of interest by an adder (20), thus obtaining an edge-emphasized signal s16.