This disclosure discloses a servomotor production method of a servomotor including a motor and an encoder. The encoder includes a rotating disk and an optical module. The rotating disk is mounted on a shaft of the motor and includes at least one concentric slit formed around a disk center. The optical module is provided with a light receiving element configured to receive light emitted from a light source and subjected to an action of the concentric slit on a substrate. The servomotor production method includes adjusting a position of the optical module with respect to the rotating disk by using the concentric slit by means of an output of the light receiving element when the optical module is fixed and arranged facing the rotating disk.