A method for detecting a phase on a gear, characterised by the method comprising: obtaining a first determination result indicating whether the gear has been detected at a first detection position, the first detection position being at a first angle relative to a reference position in a circumferential direction of a rotation axis of a spindle holding the gear; obtaining a second determination result that indicates whether the gear has been detected at a second detection position and that is different from the first determination result, the second detection position being at a second angle different from the first angle relative to the reference position in the circumferential direction; obtaining a third angle that is between the first angle and the second angle; obtaining a third determination result indicating whether the gear has been detected at a third detection position, the third detection position being at the third angle relative to the reference position in the circumferential direction; replacing the first angle with the third angle when the third determination result and the first determination result are identical to each other, or replacing the second angle with the third angle when the third determination result and the first determination result are different from each other; detecting the phase on the gear in the circumferential direction based on an angle that is between the first angle and the second angle; repeatedly obtaining the third angle and the third determination result; and detecting the phase on the gear when a difference between the first angle and the second angle is smaller than a threshold. obtaining a first determination result indicating whether the gear has been detected at a first detection position, the first detection position being at a first angle relative to a reference position in a circumferential direction of a rotation axis of a spindle holding the gear; obtaining a second determination result that indicates whether the gear has been detected at a second detection position and that is different from the first determination result, the second detection position being at a second angle different from the first angle relative to the reference position in the circumferential direction; obtaining a third angle that is between the first angle and the second angle; obtaining a third determination result indicating whether the gear has been detected at a third detection position, the third detection position being at the third angle relative to the reference position in the circumferential direction; replacing the first angle with the third angle when the third determination result and the first determination result are identical to each other, or replacing the second angle with the third angle when the third determination result and the first determination result are different from each other; detecting the phase on the gear in the circumferential direction based on an angle that is between the first angle and the second angle; repeatedly obtaining the third angle and the third determination result; and detecting the phase on the gear when a difference between the first angle and the second angle is smaller than a threshold.