An input bitstream (a first bitstream) is divided into a main code having a changeable length and the remaining data. The main code is subjected to a bit-rate conversion and combined with the remainig data to output a second bitstream. By this arrangement, the configuration ofthe bit-rate conversion circuit can be simplified and delay in the conversion process can be reduced. Incidentally, the main code corresponds to the DCT-coefficient data(e) and the quantizer scale(d), while the remaining data corresponds to the header data(a), the handling type data(b) and the motion vector data(c) respectively. Furthermore, by using the fact that if the quantizer scale is enlarged and the compression rate is raised, the degradation of a reproduced picture with a large DCT-coefficient's value of higher frequency terms is not conspicuous but the degradation of a reproduced picture with a small DCT-coefficient's value of higher frequency terms is conspicuous, and by changing the compression rate based on the DCT-coefficient's value of higher frequency terms, a bitstream with a low bit-rate but a small degradation in the reproduced picture quality can be obtained.