A servo motor controller capable of eliminating inconvenience that the position of a shaft or the like of a servo motor is detected or set erroneously due to influence of noise or the like is provided. A servo motor controller that detects Z-phase signals of an incremental-type encoder that detects rotation of a servo motor to perform driving control of the servo motor includes: a position detection unit that detects a reference position of a rotating shaft of the servo motor on the basis of the Z-phase signal of the encoder; a signal interval detection unit that measures intervals of a plurality of detected Z-phase signals; and an abnormality determination unit that determines that the Z-phase signal is abnormal when the interval measured by the signal interval detection unit is equal to or larger than a prescribed threshold.