PURPOSE: To allow continuous operation when an abnormality occurs in the compositional element of vector control.

CONSTITUTION: A V/F control system for controlling only the speed without employing any sensor is provided in addition to a vector control system for controlling the torque and speed of an induction motor 8 based on the signal from a rotational speed sensor 9 and line current sensors 18, 19. The vector control system is normally operated and a switching is made to the V/F control system upon occurrence of abnormality in the compositional element of the vector control system thus sustaining the operational control of the motor.