PURPOSE: To prevent a power cut-off accident at a part from affecting to other parts, by providing a power cut-off detecting means at a branching part connecting an optical cable to a camera, and a means that bypasses the branching part of the optical cable at the cut-off time of a power source.

CONSTITUTION: An electrical control signal inputted to an electric-light conversion device, after being converted to a light control signal, is sent out to an optical cable 12B at the next stage through an optical switch 13. When the power cut-off accident occurs for a branching part 11, a power cut-off detecting device 17 detects power cut-off, and switches the optical switch 13 to a (b) side. In this way, the light control signal inputted hereinafter from an optical cable 12A at the preceding stage is sent to the optional cable 12B at the next stage through the optical switch 13, a bypass optical cable 21, and the optical switch 13. Therefore, it is possible to perform a control operation normally for the part except the camera at the branching part where the power cut-off accident occurs, thereby, the influence of the power cut-off accident can be suppressed at the minimum level.