A control device for an AC rotating machine includes: a current detection unit; a command generation unit for generating a voltage command, using a moment of inertia J; a speed estimation unit for calculating an estimated speed ω0 on the basis of the voltage command and detected current; an output torque computing unit for calculating output torque on the basis of the detected current; and an inertia moment computing unit for calculating the moment of inertia J. The output torque computing unit gives a delay characteristic corresponding to an estimation delay of the estimated speed ω0, to the output torque, and the inertia moment computing unit calculates the moment of inertia J on the basis of the estimated speed ω0 and the output torque having the delay characteristic.