A device for controlling an alternating-current motor includes a resolver for detecting a rotational position of an alternating-current motor; and a rectangular wave voltage control unit for performing control based on an output of the resolver to provide a rectangular wave voltage to each phase of the alternating-current motor. The control unit causes an amount of change Δθ in a voltage phase of the rectangular wave voltage of each phase from a switching reference phase of each phase to increase or decrease equally for each switching in one cycle of an electrical angle determined based on the output of the resolver. In this way, a device for controlling an alternating-current motor is provided to restrain occurrence of an offset current upon rectangular wave voltage control.