A speed adjuster is provided in a belt-disc sander for controlling the rotating speed of a motor of the belt-disc sander. A speed adjust knob is disposed outside the belt-disc sander. The speed adjuster has a control circuit, a current sensor, a drive circuit, and a converter. The current sensor detects the motor current and feeds back to the control circuit, which controls the frequency variation of a SPWM signal of the drive circuit in response to the input of the speed adjust knob. The converter transforms the square wave voltage into an AC voltage for driving the motor. The desired rotating speed can be adjusted through the speed adjust knob. The control circuit will compare the present rotating speed of the motor with the set rotating speed to determine whether the present rotating speed conforms to the set one for correcting the rotating speed of the motor.