A phase change of back-scattered light generated in an optical fiber measurement method comprising: a measurement procedure executed at a plurality of times of emitting a light pulse obtained by multiplexing N, N being an integer equal to or larger than two, wavelengths into a measurement target optical fiber and measuring an in-phase component and an orthogonal component of back-scattered light generated by the light pulse in the measurement target optical fiber for each wavelength multiplexed in the light pulse; a selection procedure (SOla) of sorting the measures of the rays of the back-scattered light measured through the measurement procedure in descending order of light intensity, wherein the light intensity corresponds to a length of a vector constituted by the measured in-phase component and the orthogonal component of the back-scattered light, the rays corresponding to the respective wavelengths, and selecting a predetermined number p, p being an integer between 1 and N inclusive, of rays of the back-scattered light having the highest light intensities; a vector acquisition procedure of acquiring, for each of the p wavelengths selected through the selection procedure and at each of the plurality of times, a two-dimensional vector constituted by an in-phase component and an orthogonal component at an arbitrary position in the measurement target optical fiber among the in-phase and orthogonal components of the scattered light; a vector rotation procedure of calculating for the two-dimensional vector acquired through the vector acquisition procedure for each wavelength at a reference time a reference direction of the vector for each wavelength, and rotating the two-dimensional vector acquired through the vector acquisition procedure for each wavelength and for each other time different from the reference time by a reference rotational amount for the wavelength equal in value and in direction opposite to the calculated reference direction of the vector acquired at the reference time for each wavelength; and a calculation procedure of calculating a synthesis reference vector as an arithmetic average of the two-dimensional vector for each wavelength at the reference time, calculating a synthesis vector as an arithmetic average of the two-dimensional vector for each wavelength at the other time, which is rotated through the vector rotation procedure, and calculating a phase change amount of the back-scattered light with respect to the reference time based on an angle between the synthesis reference vector and the synthesis vector. a measurement procedure executed at a plurality of times of emitting a light pulse obtained by multiplexing N, N being an integer equal to or larger than two, wavelengths into a measurement target optical fiber and measuring an in-phase component and an orthogonal component of back-scattered light generated by the light pulse in the measurement target optical fiber for each wavelength multiplexed in the light pulse; a selection procedure (SOla) of sorting the measures of the rays of the back-scattered light measured through the measurement procedure in descending order of light intensity, wherein the light intensity corresponds to a length of a vector constituted by the measured in-phase component and the orthogonal component of the back-scattered light, the rays corresponding to the respective wavelengths, and selecting a predetermined number p, p being an integer between 1 and N inclusive, of rays of the back-scattered light having the highest light intensities; a vector acquisition procedure of acquiring, for each of the p wavelengths selected through the selection procedure and at each of the plurality of times, a two-dimensional vector constituted by an in-phase component and an orthogonal component at an arbitrary position in the measurement target optical fiber among the in-phase and orthogonal components of the scattered light; a vector rotation procedure of calculating for the two-dimensional vector acquired through the vector acquisition procedure for each wavelength at a reference time a reference direction of the vector for each wavelength, and rotating the two-dimensional vector acquired through the vector acquisition procedure for each wavelength and for each other time different from the reference time by a reference rotational amount for the wavelength equal in value and in direction opposite to the calculated reference direction of the vector acquired at the reference time for each wavelength; and a calculation procedure of calculating a synthesis reference vector as an arithmetic average of the two-dimensional vector for each wavelength at the reference time, calculating a synthesis vector as an arithmetic average of the two-dimensional vector for each wavelength at the other time, which is rotated through the vector rotation procedure, and calculating a phase change amount of the back-scattered light with respect to the reference time based on an angle between the synthesis reference vector and the synthesis vector.