PURPOSE: To conduct optional test for each node automatically by detecting a test signal sent from a frequency oscillation circuit by a frequency detection circuit.

CONSTITUTION: The loop network system is connected to nodes 1-1∼1-4 via a transmission line 2. The nodes 1-1∼1-4 apply transmission reception mutually with similar constitution. In case of the line test, a relay 32 in a sender side subscriber circuit 24 is operated by an instruction of a control circuit 27 and connected to a frequency oscillation circuit 26 to send a test signal to the circuit 24 through the transmission line 2. The relay 31 is operated in the circuit 24 by a command of the circuit 27 and connected to the frequency detection circuit 25, which detects a test signal. The result of detection is informed to the circuit 27 to test the normality of the circuit.