PURPOSE: To improve the reliability by checking a header, a sequence number and an error check value in received data at a reception station so as to discriminate the validity of the received data and making a retransmission request to a transmission station when the data are not valid.

CONSTITUTION: A transmission station A sends transmission data 1a with a header 21a, a sequence number 22a, and an error check value 23a as redundant information added to them. A reception station B discriminates the validity of the received data by checking the header 21a, the sequence number 22a, and the error check value 23a and sends an affirmative reply or a re- transmission request to the transmission station A. Thus, the transmission station A makes transmission of succeeding data or re-transmission of preceding data on a request of the reception station B. When an error takes place in the data received by the reception station B and the lapse of the occurrence of the error takes place in the opposite direction to the flow of events, the reception station B makes processing of defective reception and gives an affirmative reply to the transmission station A to allow the station A to send succeeding data.