In a transcoder, a decoder decodes a stream and an encoder encodes the stream again. The encoder calculates the quantization step value by using an average period bit rate (ASn(M)) of a first stream or an average period bit rate (ACn(M)) of a second stream. Buffers store bit rates (Sn, Cn) of the first and second streams for M periods, respectively. The encoder uses the average period bit rate (ASn(M)) of the first stream or then average period bit rate (ACn(M)) of the second stream in any one of the periods M1, M2 and M3 among M periods.