A residue code amount is calculated. A threshold act_thrsh corresponding to the residue code amount is obtained. A variable mb that designates a macro block is initialized to zero. The variable mb is compared with the total number of macro blocks. When the variable mb matches the total number of macro blocks, the process is completed. Otherwise, the rank of the activity information of the macro block designated by the variable mb is compared with the rank of the threshold act_thrsh. When the rank of the activity information is lower than the rank of the threshold act_thrsh, since there is a possibility of which q_scale can be decreased, q_scale is calculated with the activity information of the macro block mb and min_qq. q_scale of the macro block is subtracted from the total code amount. The subtracted resultant data is stored in a variable tmp. Thereafter, the variable tmp is compared with a target code amount GEN_TGT. When the variable tmp is equal to or smaller than the target code amount GEN_TGT, since the quantizing step can be fined down, the quantizing step of the macro block mb is decreased by one.