PURPOSE: To attain sure data transfer by storing new data in a data area by regarding a sequence number as a value larger than the number of sequence number storing areas, cyclically changing the data and successively transferring data optionally stored in the data areas.

CONSTITUTION: A shared RAM unit 4 is previously divided into a data storing area 6 and a sequence number storing area 5, and at the time of confirming the idle state of the area 5 in the unit 4, a transmitting unit transmits data. In this case, a sequence number cyclically using a value larger than the number of divided areas of the unit 4 is transmitted together with the transmitting data and a receiving side unit uses the sequence number for the decision of extracting order. Consequently, the data extracting order of the receiving side unit can be decided and highly reliable continuous data transfer can be attained.