A reference differential value calculation part calculates a reference differential value by subtracting an accumulated value of the target amounts of codes from an accumulated value of the amounts of generated codes from the first period to the (n−1) period. A comparison part compares a cumulative differential value obtained by subtracting the cumulative amount of generated codes from the cumulative target amount of codes of the first to m-th picture frames in the n period with the reference differential value. If the cumulative differential value is larger than the reference differential value, stuffing bits are added to the m-th picture frame to be processed by the amount of codes obtained by subtracting the reference differential value from the cumulative differential value.