To provide a motor control system for detecting a change in the load inertia of a motor at high speed and achieving fixed-position stop control at high speed even when the load inertia of the motor is changed.

After the rotational speed of a rotor of a motor M is reduced to orientation speed, a control section 17 gives a deviation between a speed designated by a position loop speed command vc and a 2 squared speed v2 to a speed controller 14. Before start of orientation control, an automatic tuning section 18 computes the acceleration of the rotor and determines a gain C of a position controller 15 based on the maximum value of the absolute value of the acceleration.