A transmitter 10B always transmits a signal (data in which a dock is embedded) generated by the serializer 11 to the communication link. The receiver 20B includes a recovery circuit 22, a deserializer 23, a selector 25, and a training signal generator 32. The training signal generator 32 generates and outputs a training signal for frequency synchronization of the recovering operation of the recovery circuit 22. The selector 25 receives the signal from the transmitter 10B via the communication link and receives the training signal output from the training signal generator 32. The selector 25 selects and outputs either the received signal or the training signal according to the level of the lock signal output from the recovery circuit 22.