PURPOSE: To achieve backup function by providing an optical receiver connected in parallel with a terminal controller and an optical transmitter optically connected with the optical receiver to transmit a command for controlling a corresponding distribution machinery.

CONSTITUTION: When a fault occurs in any one of a central controller 1, a command transmission line 2 or a terminal controller 3 and no control drive signal is fed to a distribution machinery 4, a switch 5a in an optical transmitter 5 is operated to produce a control command. Then an electrical signal suitable for transmission is produced at a code producing section 5b thereafter the electrical signal is converted through an optical signal transmitting section 5c into an optical signal 6 to be transmitted. The optical signal 6 is received at the optical signal receiving section 7a in an optical receiver 7 and converted into an electrical signal then it is decoded through a decoding section 7b into a control command, and a control drive signal is outputted from a drive output section 7c in order to perform desired control of the distribution machinery 4.