To ensure stabilized operation even if the initial value is shifted slight ly by measuring the constants of an induction motorusing the time constant of response and the offset from a target value thereby obtaining the rough value of motor constants without rotating a motor.

The current control system of an inverter is subjected to P control and the sum of primary and secondary resistances of an induction motor is estimated, along with the leakage inductance L thereof, from the current response and the offset from a converging value upon application of a current step. Since an initial value of current control gain Kp is required, a rough value of L is determined on the name plate of the motor and the Kp is set. Since the current control system is subjected to P control at the time of current response measurement under PI control, no overshoot takes place and a stabilized operation can be expected. Since the sum of primary and secondary resistances and the leakage inductance of an induction motor can be determined by measuring the step response of DC current, the motor constants of an inverter can be controlled.