An abnormality diagnosing device diagnoses a driving abnormality of a single motor driven by a plurality of motor driving devices. The motor driving devices drive the motor by supplying three-phase alternating currents to a plurality of three-phase windings possessed by the motor. The abnormality diagnosing device is equipped with a rotational speed acquisition unit for acquiring a rotational speed value of the motor, a current acquisition unit for acquiring phase current values of respective phases flowing through the three-phase windings, and a determination unit for determining the presence of an abnormality, if an absolute value of the rotational speed value exceeds a predetermined value, and an absolute value of the phase current value of any one phase of the three-phase windings is less than a threshold value continuously for a predetermined time period.