PURPOSE: To detect an optical fiber line stably with high precision by converting an analog signal of a photoelectric converting circuit to a digital signal and counting the number of bit errors and operating OR between output signals of a line unstableness detecting means and a deciding means.

CONSTITUTION: An output (e) or (f) of a digital filter 22 is inputted to a digital PLL23, a decoder 11, and an error detecting circuit 24. Continuous phase-locked clock pulses (g) having a transmission bit frequency are generated by the digital PLL23 to operate the decoder 11 and the error detecting circuit 24. A gated output (k) is inputted to a frequency divider 28, and a frequency division output (m) of the frequency divider 28, which decides NG when the number of errors exceeds a minimum allowable value, is inputted to a flip-flop 29. In the frequency divider 28, detection of error is restarted by expiration of the time set by a timer 26. An output (n) of the flip-flop 29 is supplied to an optical line unstableness detection output teminal 31 and an OR circuit 30.