PURPOSE: To sharply reduce the modifying time and man-hour of firmware by providing a firmware writing means which receives firmware data from another node station through the transmission line of a standby system and a communication controlling means and writes the received data in a memory.

CONSTITUTION: Upon receiving a firmware loading instruction command, a node station 22 transfers firmware from its own memory card 26 to a node station 23. The node station 23 rewrites a firmware area in a memory card 27 after receiving the frame. When the firmware loading on the node station 23 is completed, the node stations 22 and 23 are incorporated in a currently used system and confirmation is performed. When the firmware loading is completed to all node stations in the above-mentioned procedure, the D-filed value of the frame becomes '0' and the firmware loading work of a loop monitoring device 21 is completed.