A smoothing calculator determines a smoothed mean luminance value C1.times..mu.(t)+(1-C1).mu.'(t-1) based on a mean luminance value .mu.(t) input at time t and a smoothed mean luminance value .mu.'(t-1) read at time (t-1) from a buffer, and outputs the smoothed mean luminance value C1.times..mu.(t)+(1-C1).mu.'(t-1) as the smoothed mean luminance value .mu.(t) of the mean luminance value .mu.(t). Coefficient C1 is greater than 0 but smaller than 1. Gradation conversion responsive to luminance variation from frame to frame in a moving image is thus achieved.