An optical system improved in weight balance and stability, comprising drive mechanisms that support, movable with respect to an outer ring, a virtual rigid body consisting of an inner ring, optical element in a shape asymmetrical with respect to the optical axis, and optical element holding members for holding the optical element and fixing it to the inner ring. The inner ring, the outer ring and drive mechanisms function as a parallel link mechanism. Balance weights are provided on the virtual rigid body such that the weight of the virtual rigid body acts uniformly on the drive mechanisms. Therefore, the optical element is not likely to vibrate when subjected to an external vibration or at adjusting. The optical system is suitable for use in an exposure system requiring high exposure accuracy.