A transmission loss measurement device includes: a first monitor configured to measure a first optical power of a specific wavelength among wavelength-multiplexed light amplified by a Raman amplifier according to a power of pump light, the wavelength-multiplexed light being transmitted to an optical transmission line; a second monitor configured to measure a second optical power of the specific wavelength among the wavelength-multiplexed light received from the optical transmission line; a storage device configured to store a procedure for calculate a transmission line loss; and a processor configured to execute the procedure by: calculating a gain by the Raman amplifier, based on the power of the pump light; calculating the transmission line loss based on the first optical power, the second optical power, and the gain by the Raman amplifier; and correcting the transmission line loss according to a change of a number of wavelengths of the wavelength-multiplexed light.
