PURPOSE: To control amount of energy storage, suppress vibration and reduce vibrational noise by making inertia moment variable through moving plural weights attached movable in radial direction centering around the axis of rotation of a rotary body.

CONSTITUTION: Five cylinder blocks 10 are formed with a drum like base 10b concentric with rotary shafts 14, 16 and cylinders 10a in which pistonlike weights 2 are engaged in liquidtight movable in radial direction. Also, a rotary shaft 22 having the central axis on the axis of rotation of the base 10 is provided, and weights 2 are connected to the same via connecting rods 25 and lock pins 23, 24, and operating oil pressure spaces 18 connected to an operating oil pressure source are formed between the weights 2 and end plates 12. Therefore, it can be done to reduce vibrational noise and control the amount of energy storage freely by positioning the weights 2 in desired position with controlling operating oil pressure through detecting the position of the weights 2 with an encoder 28 of the rotary shaft 22.