Abstract: A method of controlling a rotary magnet multi-phase synchronous motor and a control system based on this method increases the permissible range of accuracy in installing a position detector on the synchronous motor and automatically calculates a phase correction to improve controllability. Using a position detector for detecting a magnet position of a rotary magnet multi-phase synchronous motor and a drive unit, the synchronous motor having the position detector is driven by a drive unit. After the synchronous motor reaches a constant speed, current and drive voltage of the synchronous motor are measured to calculate a phase correction value between a biaxial coordinate axis based upon the installation error of the position detector and a biaxial coordinate system from conversion of a polyphase AC coordinate system based on the measured current and drive voltage, constants of the synchronous motor, and the constant speed.