PURPOSE: To limit starting current by setting a speed command value to be a value according to specified start frequency, on the start, and by increasing the speed command value at a set variation speed after the current of a motor comes to a specified or lower value.

CONSTITUTION: When an operation command contact 12 is closed and the command of start is executed, the output of a speed command value according to start frequency is generated from an accelerated or reduced speed arithmetic circuit 33. By this method, from a frequency command circuit 14, the output of the frequency command value of the start frequency is generated, and from a magnetic flux command circuit 15, the output of a magnetic flux command value which is slowly increased to a value set previously at a specified rate of change is generated. After that, when motor current is reduced to a specified or lower value, the speed command value of output generated from the accelerated or reduced speed arithmetic circuit 33 is increased at the specified rate of change, and an induction motor 6 is operated at a desired speed.