The operation of a laser in an optical repeater in an optical transmission system using an optical fiber cable is telemonitored at a terminal station by changing the mark ratio of the test signal which is transmitted from the terminal station to the circuit. Each of the repeaters has an optical-electrical conversion element for the conversion of the input optical energy to electrical energy, an amplifier for amplifying the electrical output of said conversion element, a laser for converting the electrical output of the amplifier to an optical output power, a device for controlling the optical output power of the laser, a switch for providing the signal return path between the upward circuit and the downward circuit, and a monitoring circuit having a reference voltage generator for providing a reference lever which is lower than the normal signal level when the signal has the mark ratio of 1/2, and a comparator for comparing the laser output with the reference level, and a device for turning OFF the switch when the monitoring circuit provides the output signal indicating that the laser output is equal to the reference level. The terminal station designates the particular repeater and closes the switch of the designated repeater, then, transmits the test signal having the controlled mark ratio. That test signal returns to the terminal station reflected by that closed switch. By adjusting the mark ratio of the test signal, the monitoring circuit of the repeater provides the output when the laser output equals to the reference level, and has the switch turned OFF. Thus, the terminal station recognizes the degree of the degration of the laser from the mark ratio when the circuit is broken.