Motor drive device and the same use electric power steering apparatus

In a multi-system motor driving device, a power supply relay (551, 552) is interrupted when a first power supply system at a time t0 has an error. A power control unit doubles a current with an inverter (602) is supplied of a second power supply system, thereby to supplement the electric power was supplied by an inverter (601) of the first power supply system. The total output power of the inverter (601, 602) before and thus maintained after the failure. After a lapse of a predetermined time, the electric power is supplied by the second power supply system is gradually reduced, so that the inverter of the second power supply system is not operated for a long duration in order to supply at twice the power.