A motor control device for controlling the driving and regenerative operation of a motor includes CPU, ROM, RAM and a back-up RAM. An electric power is normally supplied from a subsidiary battery through a second regulator to the back-up RAM. An electric power is also supplied to the CPU, the ROM and the RAM through a main switch and a first regulator, on the one hand, and through the first regulator and a transistor which is turned ON when the number of revolutions of the motor becomes equal to or larger than a predetermined value, on the other hand. Therefore, even if the main switch is opened during traveling of a vehicle at a high speed, the counter-electromotive force of the motor can be decreased by the motor control device which is supplied with the electric power through the transistor to maintain its function, thereby preventing damage to an inverter.