A control apparatus which controls torque of an AC rotary machine based on a torque instruction value, the control apparatus comprising: - a power converter which is adapted to convert DC power to AC power and to supply the AC power to the AC rotary machine; - a current detection section which is adapted to detect a current flowing from the power converter to the AC rotary machine; and - a control section which is adapted to generate a three-phase AC voltage instruction value for controlling the AC rotary machine, to the power converter, based on the detected current detected by the current detection section and the torque instruction value, wherein the control section includes: - a current instruction calculation unit which is adapted to calculate a current instruction value; - a voltage instruction calculation unit which is adapted to calculate the voltage instruction value; and - an observer which is adapted to calculate a magnetic flux estimated value of the AC rotary machine, based on the detected current and the voltage instruction value, wherein the current instruction calculation unit is adapted to calculate current instruction values on rotational two axes, based on the torque instruction value, and the magnetic flux estimated value output from the observer, wherein the voltage instruction calculation unit is adapted to calculate the voltage instruction value, based on the current instruction values output from the current instruction calculation unit and the magnetic flux estimated value output from the observer to the voltage instruction calculation unit, and wherein the magnetic flux estimated value is a rotor-side d-axis magnetic flux estimated value. - a power converter which is adapted to convert DC power to AC power and to supply the AC power to the AC rotary machine; - a current detection section which is adapted to detect a current flowing from the power converter to the AC rotary machine; and - a control section which is adapted to generate a three-phase AC voltage instruction value for controlling the AC rotary machine, to the power converter, based on the detected current detected by the current detection section and the torque instruction value, wherein the control section includes: - a current instruction calculation unit which is adapted to calculate a current instruction value; - a voltage instruction calculation unit which is adapted to calculate the voltage instruction value; and - an observer which is adapted to calculate a magnetic flux estimated value of the AC rotary machine, based on the detected current and the voltage instruction value, wherein the current instruction calculation unit is adapted to calculate current instruction values on rotational two axes, based on the torque instruction value, and the magnetic flux estimated value output from the observer, wherein the voltage instruction calculation unit is adapted to calculate the voltage instruction value, based on the current instruction values output from the current instruction calculation unit and the magnetic flux estimated value output from the observer to the voltage instruction calculation unit, and wherein the magnetic flux estimated value is a rotor-side d-axis magnetic flux estimated value. - a current instruction calculation unit which is adapted to calculate a current instruction value; - a voltage instruction calculation unit which is adapted to calculate the voltage instruction value; and - an observer which is adapted to calculate a magnetic flux estimated value of the AC rotary machine, based on the detected current and the voltage instruction value,