The present invention is provided with: a
master station for transmitting and receiving data to and from a
control unit; and a plurality of substations for transmitting and receiving data to and from the
master station by means of a synchronous
transmission system via a common
transmission line. A control
monitoring data area, which is assigned to each of the substations in a
data transmission direction determined, and a management data area, which is different from the control
monitoring data area and is used for transmitting and receiving data to and from all of the substations, are provided. In the control
monitoring data area, an address is set as a timing for acquiring a data area allocated to the slave
station, the
master station transmits a check identifier using the management data area at a timing after execution of a process for detecting the address, and the slave
station transmits the check identifier to the slave
station when receiving the check identifier. When an operation identifier is transmitted from the master station,
data transmission and reception using the control monitoring data area are enabled, the
verification identifier is recorded and stored, and when the operation identifier is transmitted from the master station,
data transmission and reception using the control monitoring data area are enabled or disabled on the basis of the
verification result of the operation identifier and the
verification identifier.