To provide a motor driving system which can decelerate the driving of a load while consuming regenerative power with braking resistance, and in which there is no need to insulate the braking resistance at high voltage.

The motor driving system is constituted of an inverter device 10 having an inverter decelerating means 9, an AC motor 5 driven by the inverter device 10, a breaking means 6 connected to an output side of the inverter device 10 and having a transformer 61 at an input part. The inverter decelerating means 9 reduces the output frequency of the inverter device 4 so that the AC motor 5 is excited and the regenerative power of the AC motor 5 is supplied to the braking means 6 when the AC motor 5 is decelerated.