PURPOSE: To enlarge the scope of using and enhance the reliability of the damper by sensing the number of revolutions and temperature of an engine and giving an electrode an optimum voltage, which is determined by a computer so as to control the torsional vibration, thereby varying the viscosity of a magnetic fluid in order to reduce the torsional vibration.

CONSTITUTION: In a sealed chamber 5, a magnetic fluid 8 is sealed between discs 7, which are placed on an inertial body 4 side in parallel with the rotating direction of a circular plate 2, and discs 6, which are placed alternately with the discs 7 on the circular plate (2) side. Lead wires 11 and 12 are taken out of electrodes 9 and 10 of the circular plate 2 and the inertial body 4 and connected to a computer, etc., together with a temperature sensor and an engine speed sensor, both of which are installed to the engine. Based on the engine speed and the engine temperature, which are sensed by the sensors, the optimum voltage to control the torsional vibration is determined by a computer, and the voltage is given to the electrodes 9 and 10 via a power source. With this contrivance, the viscosity of the magnetic fluid 8 can be varied, thereby controlling the characteristics as a damper and reducing the torsional vibration.