To provide an ac motor controller and a control method which estimate the coil resistance value of an ac motor during its revolution, estimate the coil resistance value of the ac motor even in a high-speed rotation range, and compensate a change in the coil resistance value of the ac motor due to a temperature change during motor revolution.

The ac motor controller includes a d-axis current instruction setting unit (8) capable of arbitrarily giving a d-axis current instruction, and a coil resistance estimating unit (9) which is given at least two patterns of d-axis current instructions from the d-axis current instruction setting unit (8), calculates the inner product of a voltage instruction and a current instruction or current detection value to determine effective power per pattern, and calculates the coil resistance value of the ac motor controller from a difference between effective power per pattern and from a d-axis current instruction or d-axis current detection value.