To accelerate data transfer and to improve response performance by determining a multiplexing schedule based on the encoding rate of data in a first memory and the amount of data transferred from a transfer amount determining part, reading data from the first memory and writing them in a second memory.

When the amount of data remaining in first memories 11a-11c is less than a fixed amount, memory write parts 12a-12c transfer the fixed amount of data to the first memories 11a-11c. A transfer amount determining part 14 determines the amount of data to be transferred from the respective first memories 11a-11a so that the amount of data remaining in a second memory 13 can be more than the amount of data to be outputted from a quadrature amplitude modulator 3. Based on the encoding rate inputted from a server 1 and the data transfer amount inputted from the transfer amount determining part 14, a multiplexing control part 15 determines the read/write order (multiplexing schedule) of data to be transferred from the respective first memories 11a-11c to the second memory 13 and outputs it to a multiplexing part 16.