The present invention provides an optical receiver that includes a monitoring unit with a function to prevent the unit from misjudging of the loss-of-signal state. In particular, the present invention is applicable to the monitoring unit that receives the signal to be judged from the processing unit posterior to the optical converting unit that couples with the processing unit via the capacitor. The signal reflects the second or higher order response lag due to the coupling capacitor. The monitoring unit of the invention generates the loss-of-alarm signal when the signal is continuously smaller than the first reference by a preset time and continuously greater than the second reference by the present time.