Position-sensorless controlling method of synchronous motor

There is provided a control method capable of stably rotating a synchronous motor (3) synchronously with an output frequency even when a phase error ( DELTA ) between a real rotary axis and a control rotary axis is large. In a phase error operating unit (61), from the magnitude (Im) of the motor current, the magnitude (V1) of the motor voltage, the phase difference ( phi v) between the motor current and the motor voltage, and the motor constants (r, L), the phase difference between the motor current and the real rotary axis is determined as the first phase difference ( phi m), and the phase difference between the motor current and the virtual rotary axis is determined as the second phase difference ( phi c). The difference between the first and second phase differences is defined as the phase error ( DELTA ), and the estimation position is so modified as to make the phase error ( DELTA ) zero.