To shorten the time to decelerate and to stop in an emergency stop baking for an AC motor by fully utilizing the capacity of a common converter.

In an AC motor speed adjustable drive device for a common converter inverter, the AC motor is braked by fully utilizing a limiting capacity for braking by calculating in real time its emergency stop speed decelerating time by the limiting capacity for braking preliminarily set and the feedback speed of the AC motor, and making its decelerating time shorter than before.