A rotor is provided which is rotatably disposed between a first shell and a recording medium, in a containing space formed inside the first shell and a second shell. The rotor includes a first wall formed along the circumferential direction of the rotor, and a part of the first wall is cut out to form a first opening. The second shell includes a second wall formed so as to overlap with the inside of the first wall along the outer circumference of the recording medium, on the outside in the radial direction of the outer circumference of the recording medium. The second wall and the first wall constitute a labyrinth structure portion for isolating the recording medium and the exterior of the recording medium from each other. With the rotor rotated, the first opening and a second opening constitute a head in/out opening.