An excitation light source apparatus (1, 2, 3, 4) for outputting Raman excitation light for amplifying signal light, to a transmission path through which the signal light is transmitted, the excitation light source apparatus (1, 2, 3, 4) comprising: - an excitation light source to generate the Raman excitation light in a drive state and to stop generating the Raman excitation light in a stop state; - a light source controller (12, 312) to control intensity of the Raman excitation light in the drive state; - a light level measuring instrument to measure a light level of the signal light input into the excitation light source apparatus (1, 2, 3, 4); - a logarithmic converter to convert at least one measurement result of measuring by the light level measuring instrument to a logarithmic value; and - a main controller (15, 215, 315, 415) to decide a correction value based on the logarithmic value of the at least one measurement result in the stop state, wherein the main controller (15, 215, 315, 415) controls the light source controller (12, 312) by using the correction value and a control target value; wherein the main controller (15, 315) acquires, from the logarithmic converter, the logarithmic value of a first measurement result of the at least one measurement result in the stop state and the logarithmic value of a second measurement result of the at least one measurement result in the drive state, decides the correction value based on the logarithmic value of the first measurement result, characterized in that the main controller (15, 315) calculates a first corrected light level value by subtracting the correction value based on the logarithmic value of the first measurement result from the logarithmic value of the second measurement result, and controls the light source controller (12, 312) so that a difference between the first corrected light level value and the logarithmic value of the first measurement result gets closer to the control target value. - an excitation light source to generate the Raman excitation light in a drive state and to stop generating the Raman excitation light in a stop state; - a light source controller (12, 312) to control intensity of the Raman excitation light in the drive state; - a light level measuring instrument to measure a light level of the signal light input into the excitation light source apparatus (1, 2, 3, 4); - a logarithmic converter to convert at least one measurement result of measuring by the light level measuring instrument to a logarithmic value; and - a main controller (15, 215, 315, 415) to decide a correction value based on the logarithmic value of the at least one measurement result in the stop state, wherein the main controller (15, 215, 315, 415) controls the light source controller (12, 312) by using the correction value and a control target value; wherein the main controller (15, 315) acquires, from the logarithmic converter, the logarithmic value of a first measurement result of the at least one measurement result in the stop state and the logarithmic value of a second measurement result of the at least one measurement result in the drive state, decides the correction value based on the logarithmic value of the first measurement result, characterized in that the main controller (15, 315) calculates a first corrected light level value by subtracting the correction value based on the logarithmic value of the first measurement result from the logarithmic value of the second measurement result, and controls the light source controller (12, 312) so that a difference between the first corrected light level value and the logarithmic value of the first measurement result gets closer to the control target value. wherein the main controller (15, 215, 315, 415) controls the light source controller (12, 312) by using the correction value and a control target value; wherein the main controller (15, 315) acquires, from the logarithmic converter, the logarithmic value of a first measurement result of the at least one measurement result in the stop state and the logarithmic value of a second measurement result of the at least one measurement result in the drive state, decides the correction value based on the logarithmic value of the first measurement result, characterized in that the main controller (15, 315) calculates a first corrected light level value by subtracting the correction value based on the logarithmic value of the first measurement result from the logarithmic value of the second measurement result, and controls the light source controller (12, 312) so that a difference between the first corrected light level value and the logarithmic value of the first measurement result gets closer to the control target value.