PURPOSE: To form a transmission circuit with higher reliability by forming plural input output sections for a terminal equipment used for a bus circuit and forming bus circuits by the number.

CONSTITUTION: Duplicate input output sections 1a, 1b; 2a, 2b; 3a, 3b: 4a, 4b are provided respectively to terminal equipments 1, 2, 3, 4 and they are connected through two buses 5a, 5b. Moreover, the terminal equipments 1-4 are connected respectively to relevant terminal equipments 6-9. When a data is sent from the equipment 6 to the equipment 7, a transmission request is given from the equipment 6 to the terminal equipment 1. The terminal equipment 1 detects a line not in use at present in the buses 5a, 5b, and the transmission data is outputted to the detected line from the equipment 6. Since the data is for the terminal equipment 2, the succeeding data is sent to the equipment 7 and since the data is not for the terminal equipments 3, 4, the data are not sent to the equipments 8, 9 and they await succeeding new data.