This disclosure is directed to a data transmission system via the power supply line wherein it is characterized in that; it effectively controls branching terminal units connected to the CPU via the power supply line, by transmitting a high frequency AC signal that is mixed into the AC commercial frequency via the power supply line, while it normally surveilles the actual status of terminal units during normal operations. According to this system as a preferred embodiment of the present invention, after sampling data via the poling, a specific AC commercial frequency cyclic period can be allocated in order to properly transmit any emergency data between terminal units via a contention, and as a result, even in the slow-speed transmission line, not only any change of condition occurring in the terminal units can quickly be sent to the central processing unit, but all the control signals from the central processing unit can also be sent to respective terminal units without being held for waiting.