PURPOSE: To attain to stabilize the rotation of a rotor operated in a pressure atmosphere varied corresponding to the rotary speed of a rotor main body and to suppress the vibration thereof.

CONSTITUTION: A rotor apparatus is constituted from a bottomed cylindrical container 2, a rotor main body 3, a lid body 4, a magnetic bearing 5, a molecular pump 8 fixed to the inner peripheral surface of a side wall 7 opposed to the upper part of a peripheral surface 6, a cylinder 9 with an oval cross area concentrically arranged to the rotor 3 and an atmospheric layer 11 present in a gap 10. When the rotor 3 initiates rotation, the gas between the peripheral surface 6 and the inner surface of the cylinder 9 is exhausted from an exhaust port 11 provided to the lid body 4 by the molecular pump 8 and the pressure of the atmospheric layer 11 is reduced. Because the thickness of the atmospheric layer 11 between the cylinder 9 with an oval cross area and the peripheral surface 6 of the rotor 3 with a circular cross area becomes non-uniform along the peripheral direction of the rotor main body 3 and thick places and thin places are symmetrically present at two places, unstable vibration can be suppressed even if the rotor 3 exceeds minute vibration.