PURPOSE: To reduce the rotation variation securely even though some size variation and the like of various parts is generated owing to a little manufactural error, an abrasion of service time, and the like.

CONSTITUTION: In each revolution chamber 2 formed to a flywheel main body 1, a damper mass 5 is housed allowable to make a pendulum movement expanding to the outer side in the diameter direction while revolving. When the total mass of the damper masses 5 is made md, the total inertial efficiency is made Id, the radius is made rd, and the torque variation dimensional number of a rotation driving system to which the flywheel main body 1 is installed is made (n), the ratio of the distance R from the rotation center of the flywheel main body 1 to the fulcrums of the pendulum movements of the damper masses 5, and the distance L from the fulcrums of the pendulum movements of the damper masses 5 to the centers of gravity of the damper masses 5, R/L are set to be settled within -2%≤(R/L- R0/L0)/(R0/L0)≤+50%, or -2%≤(R/L-R0/ L0)/(R0/L0)≤+10%, to the theoretical value R0/L0= n2{1+Id/(md.rd2)}.