An optical amplifier comprising: an optical amplification section that is configured to amplify a signal light; an optical reflection medium (2, 8) which is disposed on an optical fiber connected to said optical amplification section and is capable of reflecting a noise light which exists in a predetermined wavelength range outside a signal band, among noise lights generated in said optical amplification section, to radiate the reflected noise light to the outside of a core of said optical fiber, a light receiving section that is configured to receive the noise light reflected to be radiated to the outside of the core of said optical fiber by said optical reflection medium (2, 8), to detect the power of the noise light; and a computation section that is configured to compute the total power of the noise lights generated in said optical amplification section based on the detection result of said light receiving section, wherein when said optical amplifier includes an optical filter device which transmits the signal light in accordance with a previously set transmission wavelength characteristic, on the optical fiber connected to said optical amplification section, said optical filter device is provided with a function as said optical reflection medium, and wherein said optical amplification section is configured to amplify a wavelength division multiplexed light containing a plurality of signal lights of different wavelengths, and said optical filter device is a gain equalization optical filter having a transmission wavelength characteristic capable of flattening a gain wavelength characteristic of said optical amplification section in a signal band of said wavelength division multiplexed light, and also a reflection characteristic capable of reflecting the noise light existing in the predetermined wavelength range outside the signal band to radiate the reflected noise light to the outside of the core of said optical fiber. an optical amplification section that is configured to amplify a signal light; an optical reflection medium (2, 8) which is disposed on an optical fiber connected to said optical amplification section and is capable of reflecting a noise light which exists in a predetermined wavelength range outside a signal band, among noise lights generated in said optical amplification section, to radiate the reflected noise light to the outside of a core of said optical fiber, a light receiving section that is configured to receive the noise light reflected to be radiated to the outside of the core of said optical fiber by said optical reflection medium (2, 8), to detect the power of the noise light; and a computation section that is configured to compute the total power of the noise lights generated in said optical amplification section based on the detection result of said light receiving section, wherein when said optical amplifier includes an optical filter device which transmits the signal light in accordance with a previously set transmission wavelength characteristic, on the optical fiber connected to said optical amplification section, said optical filter device is provided with a function as said optical reflection medium, and wherein said optical amplification section is configured to amplify a wavelength division multiplexed light containing a plurality of signal lights of different wavelengths, and said optical filter device is a gain equalization optical filter having a transmission wavelength characteristic capable of flattening a gain wavelength characteristic of said optical amplification section in a signal band of said wavelength division multiplexed light, and also a reflection characteristic capable of reflecting the noise light existing in the predetermined wavelength range outside the signal band to radiate the reflected noise light to the outside of the core of said optical fiber.