PURPOSE: To improve the accuracy of posture correction by providing an interferometer on a magnetic flotation type rectilinear bearing, and applying feedback to an actuator using phases of interference signals.

CONSTITUTION: There are arranged a monochromatic light source 18, a beam splitter 16 and an interferometer which has movable and fixed mirrors 12, 14 to return signals to the beam splitter 16. The movable and fixed mirrors 12, 14 of the interferometer are respectively installed on an actuator 4 and a stator 2 so that they receive interference signals from the interferometer. A control signal is therefore generated by the use of phases of plural interference signals from the detector 20, and the attitude of the actuator 4 is corrected so that those phases may become equal to each other and thereby the correction of the attitude is performed with high accuracy.