A control device for an alternating current motor according to embodiments includes a current distributor and a phase estimator. The current distributor divides a torque command by using a control phase and outputs a component that contributes to a mechanical output of the motor as a δ-axis current command and a component that does not contribute to the mechanical output as a γ-axis current command. The phase estimator computes a phase at which a γ-axis component of an addition amount of the output of the current controller and a voltage drop amount of inductance of the motor becomes zero and outputs the computed phase as the control phase.