PURPOSE: To reduce the torsional vibration of a power transmission system including a crankshaft and an output shaft effectively furthermore.

CONSTITUTION: The second mass body 8 is connected with the first mass body 2 fixed on a crankshaft 1, through a spring member 21 having high rigidity so as to turn relatively by a prescribed angle. The third mass body 4 is connected with the second mass body 8 through a spring member 14 having low rigidity so as to turn relatively by a prescribed angle, and a flywheel A is constituted. Accordingly, the spring member in action changes according to the magnitude of the input torque supplied to the flywheel A, and the inertia moment acting on the crankshaft 1 varies.