A motion-video encoding apparatus is provided with: a prediction-image generating unit for generating prediction images of blocks to be encoded, from encoded pictures or encoded areas of pictures to be encoded included in motion-video data; a prediction-error image generating unit for generating prediction error images of the blocks to be encoded, by calculating the difference between the blocks to be encoded and the prediction images; a prediction-error orthogonal transformation unit for calculating a plurality of first frequency coefficients by conducting an orthogonal transformation of the prediction error images; an prediction-image orthogonal transformation unit for calculating a plurality of second frequency coefficients by conducting an orthogonal transformation of the prediction images; a plus/minus sign evaluation unit for evaluating the plus/minus signs of the plurality of second frequency coefficients; and an adaptive encoding unit for encoding the plurality of first frequency coefficients, using plus/minus-sign matching information that indicates whether the plus/minus signs of each of the plurality of first frequency coefficients match the plus/minus signs of frequency coefficients, among the plurality of second frequency coefficients, that indicate the same frequency components as the first frequency coefficients.
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