This method includes a reference optical fiber transmission loss measurement step for measuring a reference optical fiber transmission loss measurement value, a difference value calculation step for subtracting the transmission loss reference value from the reference optical fiber transmission loss measurement value and calculating a transmission loss difference value, and a measured-optical-fiber measurement step for measuring the transmission loss of an optical fiber to be measured, the reference optical fiber transmission loss measurement step being repeatedly performed, a transmission loss difference value being calculated by performing the difference value calculation step each time a reference optical fiber transmission loss measurement value is obtained, a correction value being calculated on the basis of a plurality of transmission loss difference values, the measurement value obtained in the measured-optical-fiber measurement step being corrected using the correction value, and the transmission loss value of the optical fiber to be measured being determined.