A motor control system includes a DC-to-DC converter including a semiconductor switch and a reactor that cooperates with the switch to convert input-side DC bus voltage across first and second input-side DC buses into predetermined output-side DC bus voltage across first and second output-side DC buses and to output the output-side voltage, control circuitry that controls duty factor of the switch and determine, based on input-side detection value of the input-side voltage and output-side detection value of the output-side voltage, whether there is failure in the system when the factor is 100 percent and reactor-current detection value of reactor current through the reactor is approximately zero, a smoothing capacitor connected to the output-side buses and disposed between the output-side buses, and an inverter that is connected to the capacitor through the output-side buses, converts DC power from the output-side buses into AC power and supplies the power to a motor.