PURPOSE: To compensate disturbance by adding an equivalent disturbance including all variation factors causing influence on the operational characteristic of an object to be controlled in a speed control system to a torque command value, i.e., the input amount of the object to be controlled.

CONSTITUTION: An equivalent disturbance observer 8 for compensating for all disturbances to affect influence on an object to be controlled is arranged in a speed control system for a 3-phase induction motor 6 employing no speed sensor. An equivalent disturbance compensation output is calculated based on a torque command value T* as an input amount to the object to be controlled and an estimated rotor speed N of the motor 6, and added to the value T* of the input amount to the object to be controlled thus compensating all the disturbances. Thus, all the disturbances to cause influence on a speed control and operating characteristic of the induction motor 6 can be compensated for without requiring a speed sensor.