An optical transmission system capable of immediately stopping optical equipment which may emit a high-power output optical signal from the breaking point of a transmission line fiber. A control circuit has the first function of controlling an optical amplifier based on an optical supervisory channel received by an optical supervisory channel receiver. In addition, the control circuit has the second function of determining that the breaking of an optical fiber or the like has occurred when the strength of an optical supervisory channel received by an optical supervisory channel receiver is equal to or lower than a prescribed level and terminating the transmission of an optical signal through a line via the optical amplifier. With the second function, it is possible to prevent the emission of a high-power output optical signal from the breaking point of the optical fiber.