PURPOSE: To evade the collision of transmitted data easily and effectively, by controlling data transmission by monitoring whether the transmitted data is present or not on a bus.

CONSTITUTION: Bus monitoring receivers S are connected to the signal input terminals of delay elements 41, 42∼4n interposed in a transmission line 2 corresponding to stations 11, 12∼1n, and data transmitters T are connected to their signal output terminals. At the stations 11∼1n, said monitoring receivers S detect whether transmitted data is present on the delay elements 41∼4n, i.e., the transmission line 2 and when not, the data transmitters T send data for the 1st time.