PURPOSE: To prevent the execution of redundant resending by transmitting the inquiry of only an identifier(ID) at first, checking the ID on the receiving side, sending an answer indicating a receivable state only when double reception is not generated, and only when the answer is sent, transmitting data from the transmission side.

CONSTITUTION: A transmission processing part 1 forms a prescribed ID corresponding to each data to be transmitted, transmits the data and the ID, and in a prescribed condition, transmits the ID as the transmission of a prescribed inquiry and then a prescribed reception enabling response corresponding to the transmission is received to start the transmission of data corresponding to the ID. A reception processing part 2 stores the ID corresponding to the data at the time of normally receiving data, and when the stored ID is repeatedly received by the inquiry transmission, suppresses the transmission of the reception enabling response. Consequently, redundant resending can be suppressed and the using efficiency of a communication line can be improved.