PURPOSE: To prevent generation of overload, burning of a regenerative rotor by detecting variation in a load in an increasing direction by an increase in a real acceleration time longer than a schedule accelerating time, and switching a decelerating time to an altered decelerating time to decelerate it for longer decelerating time than the schedule decelerating time.

CONSTITUTION: If the load GD2 of a motor 2 is increased due to a certain case, the real accelerating time ta' of the motor 2 is increased longer than a schedule accelerating time ta at the rated time of the load GD2, and hence the time ta' is measured by measuring a time when the time differentiation of a speed feedback fb is not zero, and if ta<ta' is judged by a discriminator 8, an increase in the load GD2 can be sensed. In this case, a decelerating time setting to a speed controller 4 is switched by a changeover switch 9 from a second setter for setting a schedule decelerating time tβ at the rated time of the GD2 to a setter 10 for setting an altered deceleration time tβ. Thus, the decelerating rate of the motor 2 becomes smooth, and exceeding of the rated value of a regenerative resistor 3 can be suppressed by regenerative power peak value Pa.