PURPOSE: To reduce the communication time with all terminal equipments by allowing a master station to send a terminal confirmation communication command in advance and designating a terminal equipment only on a transmission line in the order of acknowledge signals from terminal equipments and allowing the terminal equipment to send a data as the normal communication.

CONSTITUTION: The master station 1 sends in advance a terminal confirmation communication command, allows terminal equipments 22∼26 to send an acknowledge signal respectively after a time in response to the size of each terminal equipment address elapses to grasp the address of the terminal equipments 22∼26 on the transmission line 7. After the normal communication command is sent from the master station 1, a virtual terminal equipment address or the like is used, only the terminal equipment on the transmission line in the order of transmission of the acknowledge signal is designated to send the data as the normal communication. Thus, since the terminal equipment not connected and the master station 1 are not communicated in the loop transmission line 7, the communication time with all terminal equipments is reduced.