In each of stages of a nonlinear filter, a successively determined target signal and plural adjacent signals are weighted-averaged with plural sets of different coefficients to calculate plural smoothed signals. Differences in level between the target signal and each of adjacent signals and signals arranged within the predetermined interval from the each of the adjacent signals are compared with a threshold value. According to the comparison, flags are set to the adjacent signal, to an adjacent signal arranged at a symmetrical position, and to adjacent signals arranged beyond the adjacent signals. The intervals of adjacent signals are different among stages. The target signal in level is used instead of the adjacent signals with smoothed signals, which are selected according to the flags. Weighting coefficients may be calculated on the basis of the comparison result, and smoothed signals may be synthesized according to the weighting coefficients.