A first stage and a second stage are disposed to face each other. A first
object is fixed on the first stage. A first displacement sensor attached
on the first stage is used to measure a distance extending from the first
displacement sensor to a plane disposed in front of the first displacement
sensor. A second object is fixed on the second stage. A second
displacement sensor attached on the second stage is used to measure a
distance extending from the second displacement sensor to a plane disposed
in front of the second displacement sensor. A moving mechanism is provided
to move one of the first stage and the second stage, with the movement of
one stage being relative to the other stage.
