When a synchronous motor (6) using a permanent magnetic field is controlled by the control of an inverter (4) receiving a pulse width modulation signal, wherein the reactive component of an armature winding current is defined as I.sub.b, the inductance of the armature winding as L, the counter electromotive force constant of the synchronous motor as K.sub.1, and the constant introduced from the pole number of the synchronous motor as K.sub.2, the value of the reactive component I.sub.b of the armature winding current is limited so that the multiplied product of K.sub.2, L, and I.sub.b does not exceed the value of K.sub.1. By the limitation, a saturation degree of the differential signal between a current command signal defined by the peak value of a reference carrier wave and the armature winding current is detected, the phase of the current command signal is changed from the phase perpendicular to the armature winding current to a rotary magnetic field main flux in response to the saturation degree, and the inverter control prevents the decrease of the output power by the pulse width modulation of the synchronous motor.