A field orientation control method for an AC motor, which includes assuming an arbitrary axis as an estimated control axis in the synchronously rotating reference frame; injecting a high-frequency fluctuating signal to the estimated control axis; determining the position of a control axis using current, voltage or impedance on the estimated control axis in the synchronously rotating reference frame, generated by the injected high-frequency signal; and determining the position and speed of the rotor based on the control axis thus located. Since this method uses the variation of impedance caused by the induction machine's generation of field and by various shapes, the control performance is more stable than other conventional control methods. This AC motor's control method may be applied to control of torque, speed and position at a low speed where it is difficult to affix a detector and where a highly efficient operation is required.