A steam turbine 20 is provided with a casing 109, a turbine rotor 25 disposed through the casing 109, and labyrinth portions 50, 55 which are disposed at the boundary between the casing 109 and the turbine rotor 25. The present invention is lightweight, buoyant, adjustable for personal preference of flotation and comfort, and can be carried on the back in a way that is similar to a back-pack. A center of a magnetic circuit which drives the holder is shifted in the outer peripheral side relative to the optical disk.