To restrain load torque of a motor to be small at acceleration or deceleration rotation when a flywheel is mounted on the drive part position of an image forming device by controlling it so as to make the moment of inertia small at rotationally driving in acceleration or deceleration and make it maximum at rotating in constant speed.

A flywheel 13 is provided with a hollow casing 14, and an electromagnet 15 is arranged in the vicinity of the center of the interior and the proper number of magnetic inertia bodies 16 slidable in the radial direction in the hollow part are provided on the outside. Hereby, when the flywheel 13 is rotated, the magnetic inertia bodies 16 are moved outward in the radial direction and moment of inertia of the flywheel becomes large, but by electrifying the electromagnet 15, the magnetic inertia bodies 16 are attracted inward in the radial direction so as to make the moment of inertia small. Consequently, when it is applied to an image forming device, by electrifying the electromagnet 15 in the acceleration territory, the moment of inertia is made small, and hence load torque of a motor can be restrained.