PURPOSE: To provide a controller for AC servo motor comprising a detector for detecting the pole position of a permanent magnet rotor and controlling the speed based on a signal from an encoder carried on the rotor shaft of a conduction controller, in which the AC servo motor can be stopped by detecting a state where the signal from the encoder is not inputted correctly.

CONSTITUTION: The controller for AC servo motor comprises a detector 4 for detecting the edge of a signal fed from a detector 2 for detecting the position of permanent magnet in a rotor, a comparing circuit 9 for comparing a rotational direction detection signal obtained from an edge detection signal with a rotational direction detection signal obtained from an edge detection signal 3 fed from an encoder 1, and a control unit 10 for making a decision that the encoder signal is abnormal if more than one pulse signal is not obtained from the encoder signal between two pulse signals of rotational direction detection signal obtained from pole position detection signal and receiving a signal for stopping a servo motor 11.