PURPOSE: To prevent a change in picture quality which is uncomfortable to eyes from being caused in a picture data by devising the method such that a quantization step size is not revised when a in-frame encoding processing in a forced refresh mode is designated while the said mode is repeated at a prescribed period.

CONSTITUTION: A quantization control unit 36 discriminates whether or not a type of a processing object macro block is of the forced refresh. When the answer is affirmative, the quantization control unit 36 sets a preceding frame quantization step size PQNT used for the quantization processing as a quantization step size PQNT. Then the quantization is executed by the same quantization step size as that of the preceding frame. Thus, when the forced refresh processing is executed, a change in the picture quality practically not causing unpleasant observation is avoided.