To apply braking effect to inertia rotation of an inertia rotor supported in a comparatively small space.
A brake band 22 is provided in such a manner as being capable of making contact with/separating from the circumferential surface of an inertia rotor 21 while circling along its surface and braking action is applied by tightening this band. A support shaft 24 is provided rotatably on a base stand 23 provided near the peripheral surface of a rotor and having one end of the band connected thereto. A contact roller 26 making touching rotation on the peripheral surface of a rotor and a rotary roller 27 whose one part in the peripheral direction of the peripheral surface is composed of a magnet 27a or magnetic material while the rest thereof is made of non-magnetic material are fixed to the support shaft. A braking member 28 movable between a non-braking position for releasing the band along the peripheral surface of a rotary roller and a braking position for tightening it and whose curved surface is made of a magnet or a magnetic material and a guide member 29 for guiding the braking member to a non-braking position and a braking position. A part of the rotary roller is composed of a magnet, and curved surface preferably includes a reverse magnetic polarity and a contact roller is preferably a rubber roller.