A screw decanter centrifuge including a cylindrical bowl rotatable in one direction, a main drive motor coupled to the bowl for rotatably driving the bowl, a screw conveyor which is disposed concentrically in the bowl and which is rotatable in the same direction as the bowl at a differential rotation speed from the bowl, and an induction type screw conveyor motor for driving the screw conveyor. The differential rotation speed is changed in response to a carrying torque of the screw conveyor. An induction type screw conveyor motor is provided for driving the screw conveyor. The induction type screw conveyor motor has a shaft connected directly to, or coupled to, a shaft of the screw conveyor, and the induction type screw conveyor motor is controlled by a speed-torque controller. The speed-torque controller includes a speed controller for calculating the differential rotation speed based on rotations indicated by an induction motor tachometer for detecting a speed of the induction type screw conveyor motor, and on rotations indicated by a bowl tachometer for detecting a speed of the bowl. The speed-torque controller further includes a vector control inverter for independently controlling an exciting current and a torque current of the induction type screw conveyor motor based on a speed command value outputted from the speed controller and an actual speed of the induction type screw conveyor motor outputted from the induction motor tachometer.