An input monitoring system is provided in an optical amplifying repeater to monitor a level of an optical input signal. A monitored level of the optical input signal is used to detect a fault on an optical transmission path or to control a bias current for a laser diode which emits and supplies an exciting signal for exciting an optical fiber amplifier. To accurately monitor a level of the optical input signal, the input monitoring system comprises an input terminal supplied with the optical input signal, an optical fiber amplifier for amplifying the optical input signal, a first photo diode for detecting and outputting an electric signal corresponding a level of the optical input signal, and an optical switch operatively connected to the input terminal, the optical fiber amplifier and first photo diode for switching the optical input signal alternatively into the optical fiber amplifier and the first photo diode.