To provide a motor controller and its controlling method which detect a failure mode without lowering the accuracy of zero-point adjustment in inverter control and make it possible to run a vehicle by other motor according to the failure state.

A vehicle driving device 1 includes a battery 15 which is a DC power source, a converter 14 for boosting the voltage of the battery 15, an inverter 28 connected to an MG1 motor 11 for driving a vehicle, an inverter 29 connected to an MG2 motor generator 12 which works as a motor or a generator, and the motor controller 18 for controlling a motor. The vehicle driving device 1 is provided with voltmeters, non-energization determination devices 13 and 14 for determining whether or not the inverters 28 and 29 are in a non-energized state, and ammeters 41-44 for detecting a current applied to the motor from each phase arm of the inverters. Signals from the voltmeters and the ammeters and signals from the non-energization determination devices 13 and 14 are sent to the motor controller 18.