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This article reports an active air bearing that utilizes air films generated by ultrasonic oscillation. The films on the bearing surfaces of a horn, which is a stator, lift and restrain the rotor. Moreover, the oscillation that modulates the amplitude via the positioning signal varies the thickness of the air film. Thus, this bearing regulates the position of the rotor using a PI controller with a displacement sensor so that its runouts become zero. Furthermore, no pneumatic plant is needed in this bearing. The repetitive runouts were 0.23 μm p-p in an axial direction and 1.0 μm p-p in a radial without compensation. Nonrepetitive runouts were 0.061 μm 3σ in an axial direction and 0.065 μm 3σ in a radial without compensation. PI feedback control, in which a fiber optic displacement sensor measures the movement of a master ball mounted on the rotor, decreased repetitive axial runout from 0.232 μm p-p to 0.118 μm p-p. The settling time for 0.2 μm step positioning in an axial direction was less than 0.2 s. The positioning resolution was less than 40 nm. Infinite stiffness under a 1.2 N static load was obtained in the experiments. © 2004 American Institute of Physics.