The minimum Sum of Absolute Differences obtained by a motion vector search roughly judges the magnitude of quantization error by whether or not exceeding a predetermined threshold value. When the quantization error is lower, whether or not visually noticeable noise exists in some of the pixels of the current macroblock is judged based on the amount of flatness and noise detected in each of the 4x4 pixel blocks of the current macroblock partitioned into 16 sub-macroblocks. If there is visually noticeable noise, intra-frame coding is selected. When the quantization error is higher, whether or not visually noticeable noise exists in the current macroblock is judged while considering the magnitude of the motion vector. If there is visually noticeable noise, intra-frame coding is selected.