An optical transmission system is provided which is capable of booting the system by using an ASE (Amplified Spontaneous Emission) light in a manner to provide high quality. A pre-amplifier receives noise light and does gain setting in an amplifier booting mode and receives an optical signal and to amplify the received signal in a working mode. A variable dispersion compensator is placed in a preceding-stage of the pre-amplifier and makes compensation of dispersion of a wavelength occurring when an optical signal propagates through an optical fiber in a variable manner. A switching unit to select, when the noise light passes through the variable dispersion compensator, if a level of the noise light is deviated from a desired level, a bypass route so as to make the noise light not be input to the variable dispersion compensator but be input to the pre-amplifier in an amplifier booting mode according to a switching instruction and to select a passage route so as to make the optical signal, after having been input to the variable dispersion compensator and having undergone dispersion compensation, be input to the pre-amplifier according to the switching instruction in a working mode.