To improve reliability by preventing a fault and breakage beforehand by replacing the field current adjusting circuit of a DC motor to a chopper so as to eliminate a movable part.

This speed control device of the DC motor 30 performs the speed control of the DC motor 30 by feeding the output voltage of a DC generator 20 to an armature of the DC motor 30, and comprises the field current adjusting means of the DC generator 20. The filed current adjusting means is constituted of the chopper 60, more specifically, a quadrant chopper circuit 606 composed of semiconductor switches of vertical arms that correspond to two phases in a main circuit of a three-phase voltage type inverter for three-phase AC motor driving.