PURPOSE: To relax an amount of programs of the said loading and the condition of a transfer time by using a spare transmission line without disturbing the communication in operation on an active transmission line at the initial program loading.

CONSTITUTION: When power is applied, a power-on detection signal S resets a node device Nd and informs a power-on signal S3 to a changeover control circuit 9. Further, a supervisory control section 5 detects which of transmission lines 21, 22 is active and informs it to the changeover control circuit 9 via an active transmission line command signal line 102. When the transmission line 21 is active, a changeover circuit 6 selects the transmission line 22. Then a communication control section outputs an initial program loading request and the said loading is executed in a route of the control section 7, the circuit 6, a transmission and reception device 42, an accessor 32 and the transmission line 22. When the loading is finished, the communication control section 7 transmits an end signal S2 to the circuit 9 and the changeover circuit 6 selects the transmission line 21. Then the state reaches the normal communication state.