PURPOSE: To reduce the burden of the master station as well as to ensure an effective control of the slave stations by the group-based process to the slave stations on the same circuit to be connected to the main station.

CONSTITUTION: Plural units of slave stations are connected to master station 11 by sharing signal line 15 and then divided into groups containing more than one identification address of the slave station. And the address identifying the groups is memorized in group address register 17. At the same time, each slave station within the group is memorized in individual address register 21 after the correspondence given to the bit position. Both register 17 and 21 are decided rewritable by station 11, and the addresses corresponding to the slave stations within the groups are produced in sequence at control logic circuit 22 by the command given from station 11 and based on the contents of register 17 and 21. Based on these addresses, the polling is given to the salve stations.