A control apparatus for an AC rotating machine that can reliably and stably start up an AC rotating machine, particularly a synchronous motor using a permanent magnet, in position-sensorless vector control thereof, includes: a steady speed calculator that calculates, during steady state control of the AC rotating machine, a rotation angular frequency of the AC rotating machine based on an AC current and voltage commands; and a start-up speed calculator that calculates, during start-up control within a predetermined period after the AC rotating machine has been started up, the rotation angular frequency of the AC rotating machine based on the AC current and the voltage commands. The control apparatus corrects during the start-up control current commands so that the AC voltage amplitude of the voltage commands will be a constant value not more than the maximum output voltage of a power converter.