PURPOSE: To improve the torque utilization factor of a motor by providing acceleration function generating means and deceleration function generating means in a speed command circuit for a motor driving device.

CONSTITUTION: A speed set signal from a speed setter 2 is input to an acceleration/deceleration calculator 3 composed of an integrator, and converted to a speed command signal which is linearly varied at a predetermined timewise change rate. An acceleration function generator 4 and a deceleration function generator 5 are connected to the rear stage of the calculator 3, and either one of the generators 4, 5 is selected by a signal switching unit 6 switched corresponding to the output signal from an acceleration/deceleration discriminator 7. When the characteristics of the generator 4 or 5 is so selected as to be adapted for the torque characteristic of a load coupled to a motor, the torque utilization factor of the motor is improved.