A motor drive device includes a bridge circuit 10 having a plurality of legs with switch elements 101 in upper and lower arms. High frequency AC voltage output from the bridge circuit 10, when it is connected to an AC power supply 1, is converted into a DC voltage via a transformer 21 and an AC/DC conversion circuit 11, thus charging the battery 2. Also, the DC voltage of the battery 2 is converted into an AC voltage by the bridge circuit 10 when the AC power supply 1 is cut off from the bridge circuit 10, and supplied to the alternating current motor 3. Because of this, the isolating transformer 21 can be driven at high frequency while the motor driving power conversion circuit is also used for battery charging, and a reduction in size and reduction in cost of the overall device are achieved.