Digital signals having respective pieces of frequency information different from each other are bundled, and transmitted at high speed. On receiving side, digital signals retaining the respective pieces of frequency information are recovered and separated. Transmitter apparatus divides pieces of transmission data that have the different pieces of frequency information and correspond to respective input channels into data blocks having a fixed length, as valid data, and subsequently multiplexes the data blocks corresponding to the respective input channels and outputs the multiplexed data to a transmission path. A receiver apparatus divides data string received into data flows and subsequently restores the transmission data, from the data blocks consecutive in each data flow and stores the restored data, and outputs transmission data corresponding to the respective data flows in synchronization with clocks generated for these data flows.