The present invention improves the calculation accuracy of the phase of gear teeth. This method for calculating the phase of gear teeth having Z number of teeth involves: a gear teeth amplitude signal acquisition step for acquiring a gear teeth amplitude signal (S(c)) in which the angle (c) of the gear teeth and a value corresponding to the irregularities on the outer circumference of the gear teeth at said angle (c) are associated with one another for at least one revolution of the gear teeth; a phase calculation step for calculating the phase (B) of the angular pitch (P) of the gear teeth in accordance with the number (Z) of teeth when the gear teeth amplitude signal (S(c)) was subjected to frequency decomposition; and a teeth alignment angle calculation step for calculating the teeth alignment adjustment angle on the basis of the phase (B) detected by means of a phase calculating unit.
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