PURPOSE: To improve the reliability by providing a request serial data extraction section to a sender side control unit and providing a data validity check section to a receiver side control unit.

CONSTITUTION: While a data application management section 2 provided to a sender side control unit 1 stores each data while providing a data serial number, a request serial data extraction section 3 is provided to extract a data requested with a data serial number and sends the data together with the data serial number, a receiver side control unit 5 uses a data validity check section 8 to collate the request data serial number and the received data serial number of the received data thereby confirming the reception of the request data. Thus, the data is requested in the order of data serial number and missing data and duplicate transmission of data are prevented by collating the serial number with the received data serial number.