PURPOSE: To improve the transmission efficiency and to prevent data transmission from being stopped due to the occurrence of a fault by allowing each equipment to judge whether it is the time for the equipment of its own to transmit a data based on a received equipment number received while counting the timing from a received point of time of the data from other equipment and to transmit sequentially the equipment number and data of its own equipment.

CONSTITUTION: A specific quipment at the leading transmits the equipment number of the equipment of its own and data to a transmission line and the transmitted equipment number decides an equipment to transmit the data next. Then the equipment counts a prescribed timing and transmits the equipment number of the equipment of its own and data and then the succeeding equipment is decided based thereupon. Since the necessity of polling from a master set to each equipment is eliminated, the transmission efficiency is improved. If a fault takes palce due to any cause and the data of the equipment No.1 is not transmitted, the equipment No.2 counts a prescribed timing, confirms if the data of the equipment No.1 receives no data, counts this time a decided timing and transmits the equipment number of the equipment of its own No.2 and the data.