A controller for an AC rotary machine, the controller comprising: current vector detection means adapted to detect a current vector of the AC rotary machine and output the current vector as a detected current vector; AC rotary machine control means adapted to refer to an estimated magnetic flux phase and output a voltage instruction vector such that the detected current vector coincides with a current instruction vector; voltage application means adapted to apply a voltage to the AC rotary machine based on the voltage instruction vector; adaptive observation means adapted to output the estimated magnetic flux phase based on the voltage instruction vector; and magnetic flux vector detection means adapted to detect a magnetic flux vector and output the magnetic flux vector as a detected magnetic flux vector, the controller being characterized in that: the adaptive observation means are adapted to output an estimated current vector and an estimated magnetic flux vector in addition to the estimated magnetic flux phase, based on a current deviation vector and an amplified deviation vector in addition to the voltage instruction vector, wherein the estimated current vector and the estimated magnetic flux vector are derived from the voltage instruction vector and the amplified deviation vector by subtracting, integrating, and calculating, and the estimated magnetic flux phase is derived from the estimated magnetic flux vector by calculating; and the controller further comprises deviation vector calculation means adapted to calculate the current deviation vector which is a deviation between the estimated current vector and the detected current vector and a magnetic flux deviation vector which is a deviation between the estimated magnetic flux vector and the detected magnetic flux vector and to output the calculated current deviation vector and the calculated magnetic flux deviation vector as the amplified deviation vector by means of a variable gain amplifier in dependence of the estimated speed, the estimated speed being derived from the adaptive observation means. current vector detection means adapted to detect a current vector of the AC rotary machine and output the current vector as a detected current vector; AC rotary machine control means adapted to refer to an estimated magnetic flux phase and output a voltage instruction vector such that the detected current vector coincides with a current instruction vector; voltage application means adapted to apply a voltage to the AC rotary machine based on the voltage instruction vector; adaptive observation means adapted to output the estimated magnetic flux phase based on the voltage instruction vector; and magnetic flux vector detection means adapted to detect a magnetic flux vector and output the magnetic flux vector as a detected magnetic flux vector, the controller being characterized in that: the adaptive observation means are adapted to output an estimated current vector and an estimated magnetic flux vector in addition to the estimated magnetic flux phase, based on a current deviation vector and an amplified deviation vector in addition to the voltage instruction vector, wherein the estimated current vector and the estimated magnetic flux vector are derived from the voltage instruction vector and the amplified deviation vector by subtracting, integrating, and calculating, and the estimated magnetic flux phase is derived from the estimated magnetic flux vector by calculating; and the controller further comprises deviation vector calculation means adapted to calculate the current deviation vector which is a deviation between the estimated current vector and the detected current vector and a magnetic flux deviation vector which is a deviation between the estimated magnetic flux vector and the detected magnetic flux vector and to output the calculated current deviation vector and the calculated magnetic flux deviation vector as the amplified deviation vector by means of a variable gain amplifier in dependence of the estimated speed, the estimated speed being derived from the adaptive observation means. the adaptive observation means are adapted to output an estimated current vector and an estimated magnetic flux vector in addition to the estimated magnetic flux phase, based on a current deviation vector and an amplified deviation vector in addition to the voltage instruction vector, wherein the estimated current vector and the estimated magnetic flux vector are derived from the voltage instruction vector and the amplified deviation vector by subtracting, integrating, and calculating, and the estimated magnetic flux phase is derived from the estimated magnetic flux vector by calculating; and the controller further comprises deviation vector calculation means adapted to calculate the current deviation vector which is a deviation between the estimated current vector and the detected current vector and a magnetic flux deviation vector which is a deviation between the estimated magnetic flux vector and the detected magnetic flux vector and to output the calculated current deviation vector and the calculated magnetic flux deviation vector as the amplified deviation vector by means of a variable gain amplifier in dependence of the estimated speed, the estimated speed being derived from the adaptive observation means.