To execute slip compensation, without execution of feedback control and use of a complicated read circuit, by setting a ratio of a magnetic flux component current to a torque component current to a particular value in the direction of a primary current to calculate a slip compensation value depending on an amplitude of the primary current of an induction motor.

In a speed controller of an induction motor, which executes full-wave rectification for a 3-phase power source with a converte 1 and smoothing the rectified power source with a capacitor 2 to connect to an inverter 3 and also comprises an arithmetic unit 6 and a PWM pulse generator 7 operating depending on a frequency command 5, a current value detected by a power source detector 8 is input to the arithmetic unit 6. Here, a compensating value for the slip frequency of the induction motor 4 is calculated by setting a ratio of the magnetic flux component current to a torque component current to 3:7, under the condition that the magnetic flux component current shall not change rapidly for the change of a load and the magnetic flux component current is equal to the rated magnetic flux component current. Thereby, slip compensation which is sufficiently reliable for practical use may be realized very easily.