A statistical value calculation part specifies macroblocks positioned around an object macroblock and calculates a minimum average value of activities of the macroblocks. When images of the macroblocks are flat and the minimum average value is smaller than an activity of the object macroblock, the minimum average value is set as an adjustment value. A correction factor determination part determines a correction factor on the basis of the adjustment value and a factor determination table. By multiplying a reference quantization step value by the correction factor, a quantization step value of the object macroblock is determined. Since the quantization step value reflects a distribution of the activities of the macroblocks, it is possible to suppress a local change of the quantization step value.