A motor drive device in which the torque vector of the motor is controlled so that the correlation between the number of pulses of a clock signal and the phase of the torque vector of the motor is held in common for multiple excitation methods. The excitation method having the greatest number of steps is used as a reference, and the torque vector of the motor is maintained in the same phase as before switching when the excitation method is switched. The torque vector is in the closest phase in the rotation direction of the motor when there is no phase that is the same as the phase prior to switching.