A motor driving system with a pair of units connected in series to form a DC power supply with an intermediate potential connection point, positive and negative electrode lines connected to the positive and negative sides of the power supply through positive and negative electrode side switching circuits, an intermediate potential line with one end connected to the intermediate potential point, a DC to DC converter unit having two switching devices connected in series between the positive and negative electrode lines, a DC link circuit connected in parallel to the DC to DC converter unit between the positive and negative electrode lines, and a DC to AC converter unit driving an AC motor, the other end of the intermediate potential line connected to the connection point between the switching devices with a reactor in the intermediate potential line or in each of the positive and negative electrode lines.