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.