To miniaturize a controller and to reduce its cost by absorbing generated power with a system other than generator braking or regenerative braking and to solve long deceleration time that is caused by low deceleration rate by a software system.

In variable speed controller provided with an automatic voltage controlling part 10 for holding the output voltage of an inverter 2 in a state of a constant V/f ratio against fluctuation of a DC voltage, the speed deceleration state of an electric motor is detected from the rise of a DC intermediate voltage by the comparing part 113A of an output voltage stressing circuit 13. This detection causes a changeover part 13B to raise the output of the automatic voltage controlling part 10, thereby increasing the output voltage of the inverter. As a result, the efficiency of power conversion in the electric motor and inverter is lowered and, by increasing the power consumed by the motor and inverters, the voltage rise of the DC intermediate circuit is suppressed. This invention includes detection of the speed deceleration state of the motor from its effective power and the direct raise of the current setting signals of the inverter by the detection of the speed deceleration state.