To attain improvement in the reliability of cyclic transmission switching to a slave system in the case of master system fault by providing a mechanism for switching the setting area of a shared memory at another station to a transmission area, reception area or transmission/reception stop area.

A shared memory 6 writes/reads data from a host computer 4 of a present station 2 or writes data received from another station 2. A table 7 for transmission/reception setting stores the setting information of transmission area, reception area and transmission/reception stop area for each block of the shared memory 6. In this case, transmission/reception setting information is added for controlling the data transmission/reception operation of the other station 2 to transmission data by the instruction of the host computer 4 so as to transmit data to the shared memory area set to the transmission area by the other station 2. At the station to receive the data to which this transmission/reception setting information is added, this information is fetched and the data transmission/reception operation of the present station 2 is changed.