To provide an optical transmission system which is sufficiently protected even when 'disconnection' occurs on an optical line and which also has a simple system structure and whose facility cost is inexpensive.

This optical transmission system performs multiple node connections of optical transmission devices 10A and 10B through 1st and 2nd bidirectional lines 11 and 12 which consist of optical transmission lines that send bidirectional optical data in a single optical fiber by using a bidirectional optical module. In such cases, one direction of the line 11 is used as a current line that performs data insertion, and the other direction is used as a protection line. Also, one direction of the line 12 is used as a protection line and the other direction is used as a current line. When the line 11 is disconnected, data of the line 11 is inserted into the protection line of the line 12 and transmitted, and when the line 12 is disconnected, data of the line 12 is inserted into the protection line of the line 11 and transmitted.