To provide a control method of synchronous motor in which good response and robustness can be attained without increasing the resolution of an encoder even when the sampling time is shortened.

In the control method of a synchronous motor providing actual voltage commands Vuref,... appropriate to a power conversion circuit such that the synchronous motor approaches an actual command θref, mechanical control operation is performed based on the actual command θref and the actual position θ and the estimated speed w of a rotor, a torque command Tref is provided, synchronous motor currents Iu, Iv and Iw are converted into the d-axis actual current Id and the q-axis actual current Iq on the rotational coordinate axis rotating in synchronism with a rotor flux vector using the actual position θ of the rotor, operation is performed based on the actual position θ, the torque command Tref, the d-axis actual current Id and the q-axis actual current Iq, a d-axis voltage command Vdref and a q-axis voltage command Vqref are provided, the d-axis voltage command Vdref and the q-axis voltage command Vqref are converted into actual voltage commands Vuref,... using the actual position θ, and a speed w is estimated based on the q-axis actual current Iq and the q-axis voltage command Vqref.