PURPOSE: To transfer alarm information to a down-stream side by superimposing the alarm information onto a stimulated light source as soon as interruption of an input signal light is detected and a standby light source is selected.

CONSTITUTION: When an input optical signal is interrupted, a light receiving element 3 and an input interruption detection circuit 4 detect the input interruption and input interruption information from the input interruption detection circuit 4 is inputted to a frequency modulation circuit 8 and a changeover circuit 7. Upon the receipt of the input interrupt information, the frequency modulation circuit 8 generates a digital code pattern in response to the alarm information including a specific number of each optical direct amplifier and converts it into an FSK signal. Furthermore, the FSK signal is inputted to a stimulation semiconductor laser drive circuit 9, which applies light intensity modulation to a stimulation semiconductor laser 10.