A fine (short-stroke) stage is mounted on an XY slider and is capable of
being moved over a minute range in the XY directions. Though the XY slider
is moved along the X direction by a Y guide bar, movement along the X
direction is decided by an X guide bar. If the relative angle between the
direction of the X guide bar and the surface of a bar mirror of the fine
stage is represented by .alpha., then the X guide bar will be moved by
.alpha.x when the XY slider has been moved by x along the X direction by
the Y guide bar. After the fine stage is thus positioned, the relative
positional relationship between the fine stage and the XY slider is held
constant.
