A phase measurement method for observing temporal change of the phase of backscattered light generated in a measurement target optical fiber, comprising: measuring an in-phase component and an orthogonal component of scattered light generated at a wavelength-multiplexed light pulse incident on a measurement target optical fiber; acquiring, at a plurality of times, for each wavelength multiplexed in the light pulse, a two-dimensional vector constituted by the in-phase component and the orthogonal component of the scattered light at an arbitrary position in the measurement target optical fiber; the method being characterized in comprising: calculating for the two-dimensional vector of each wavelength at a reference time a reference direction of the two-dimensional vector for each wavelength and rotating the two-dimensional vector of each wavelength at another time which is different from the reference time by a reference rotational amount for the wavelength equal in value and in a direction opposite to the calculated reference direction of the vector acquired at the reference time for each wavelength; and calculating a synthesis reference vector as an arithmetic average of the two-dimensional vector of each wavelength at the reference time, a synthesis vector as an arithmetic average of the two-dimensional vector of each wavelength at the other time, and a phase change amount of the scattered light based on an angle between the synthesis reference vector and the synthesis vector. measuring an in-phase component and an orthogonal component of scattered light generated at a wavelength-multiplexed light pulse incident on a measurement target optical fiber; acquiring, at a plurality of times, for each wavelength multiplexed in the light pulse, a two-dimensional vector constituted by the in-phase component and the orthogonal component of the scattered light at an arbitrary position in the measurement target optical fiber; the method being characterized in comprising: calculating for the two-dimensional vector of each wavelength at a reference time a reference direction of the two-dimensional vector for each wavelength and rotating the two-dimensional vector of each wavelength at another time which is different from the reference time by a reference rotational amount for the wavelength equal in value and in a direction opposite to the calculated reference direction of the vector acquired at the reference time for each wavelength; and calculating a synthesis reference vector as an arithmetic average of the two-dimensional vector of each wavelength at the reference time, a synthesis vector as an arithmetic average of the two-dimensional vector of each wavelength at the other time, and a phase change amount of the scattered light based on an angle between the synthesis reference vector and the synthesis vector. calculating for the two-dimensional vector of each wavelength at a reference time a reference direction of the two-dimensional vector for each wavelength and rotating the two-dimensional vector of each wavelength at another time which is different from the reference time by a reference rotational amount for the wavelength equal in value and in a direction opposite to the calculated reference direction of the vector acquired at the reference time for each wavelength; and calculating a synthesis reference vector as an arithmetic average of the two-dimensional vector of each wavelength at the reference time, a synthesis vector as an arithmetic average of the two-dimensional vector of each wavelength at the other time, and a phase change amount of the scattered light based on an angle between the synthesis reference vector and the synthesis vector.