A method of detecting a positional change amount due to a movement of a moving body by reading, with a sensor, a plurality of graduations disposed along the direction of the movement, the method comprising: a) a step of acquiring from the sensor two pseudo sine wave signals that are 90° out of phase with each other, depending to the positional change amount, wherein each pseudo sine wave signal is set such that the amount of one graduation of the plurality of graduations is one cycle; b) a step of performing Fourier transform to each pseudo sine wave signal in the range of at least one graduation, and from a spectrum intensity of each frequency component obtained by the Fourier transform, calculating a signal intensity of a fundamental wave component and a signal intensity of at least one harmonic component; c) a step of calculating a gain corresponding to each of the at least one harmonic component by dividing the signal intensity of each of the at least one harmonic component by the signal intensity of the fundamental component; and d) a step of detecting the positional change amount by subtracting from each of the two pseudo sine wave signals each harmonic component multiplied by the corresponding gain and performing arctangent operation to one obtained by dividing one pseudo sine wave signal having a phase delayed by 90° of the two subtracted pseudo sine wave signals by the other pseudo sine wave signal, characterized in that the step d) comprising: d1) a step of setting, for each harmonic component, a plurality of provisional phase differences with respect to the fundamental wave component, being an ideal sine wave signal, which is set such that one of the plurality of graduations is the origin and the amount of one graduation is one cycle; d2) a step of calculating a provisional positional change amount by subtracting from each pseudo sine wave signal each harmonic component, including one of the plurality of provisional phase differences, multiplied by the corresponding gain; d3) a step of calculating an ideal positional change amount of the ideal sine wave signal; d4) a step of calculating a positional error by subtracting from the provisional positional change amount the ideal positional change amount; d5) a step of calculating a positional error amplitude in each provisional phase difference by extracting a maximum positional error that is the maximum and a minimum positional error that is the minimum among the positional errors and subtracting from the maximum positional error the minimum positional error; d6) a step of determining as a true phase difference a provisional phase difference having the smallest positional error amplitude among the respective positional error amplitudes calculated for the plurality of provisional phase differences; and d7) a step of subtracting from each pseudo sine wave signal each harmonic component, including the true phase difference, multiplied by the corresponding gain. a) a step of acquiring from the sensor two pseudo sine wave signals that are 90° out of phase with each other, depending to the positional change amount, wherein each pseudo sine wave signal is set such that the amount of one graduation of the plurality of graduations is one cycle; b) a step of performing Fourier transform to each pseudo sine wave signal in the range of at least one graduation, and from a spectrum intensity of each frequency component obtained by the Fourier transform, calculating a signal intensity of a fundamental wave component and a signal intensity of at least one harmonic component; c) a step of calculating a gain corresponding to each of the at least one harmonic component by dividing the signal intensity of each of the at least one harmonic component by the signal intensity of the fundamental component; and d) a step of detecting the positional change amount by subtracting from each of the two pseudo sine wave signals each harmonic component multiplied by the corresponding gain and performing arctangent operation to one obtained by dividing one pseudo sine wave signal having a phase delayed by 90° of the two subtracted pseudo sine wave signals by the other pseudo sine wave signal, d1) a step of setting, for each harmonic component, a plurality of provisional phase differences with respect to the fundamental wave component, being an ideal sine wave signal, which is set such that one of the plurality of graduations is the origin and the amount of one graduation is one cycle; d2) a step of calculating a provisional positional change amount by subtracting from each pseudo sine wave signal each harmonic component, including one of the plurality of provisional phase differences, multiplied by the corresponding gain; d3) a step of calculating an ideal positional change amount of the ideal sine wave signal; d4) a step of calculating a positional error by subtracting from the provisional positional change amount the ideal positional change amount; d5) a step of calculating a positional error amplitude in each provisional phase difference by extracting a maximum positional error that is the maximum and a minimum positional error that is the minimum among the positional errors and subtracting from the maximum positional error the minimum positional error; d6) a step of determining as a true phase difference a provisional phase difference having the smallest positional error amplitude among the respective positional error amplitudes calculated for the plurality of provisional phase differences; and d7) a step of subtracting from each pseudo sine wave signal each harmonic component, including the true phase difference, multiplied by the corresponding gain.