A magnetic-pole detecting system for synchronous AC motors and a magnetic-pole detecting method therefor are provided. The magnetic-pole detecting system for synchronous AC motors includes a position detector 10 for detecting the relative position between the moving element 3 and the stator of an AC motor 5; a phase-update command unit 52 for making a plurality of currents with different phases flow in coils 3a and 3b so that the moving element 3 moves to a plurality of stable points; a reversal command unit 56 for making currents flow in the coils 3a and 3b so that the moving element 3 reverses from the previous direction in which the moving element 3 has been moved by the phase-update command unit 52; a current cut-off command unit 54 for cutting off the currents flowing in the coils 3a and 3b when the position-detector 10 detects a movement of the moving element 3; a reversal-determining unit 64 for determining, based on a position-detecting signal from the position-detector 10, a direction in which the moving element has moved based on the phase-update command unit 52, and for determining that the determined direction has reversed between the previous and the present instances; and a stable-point-simulator for simulating a stable point by means of the phase of the currents flowing in the moving element 3 when the reversal-determining unit 64 has detected the reversal.