At a first time, a first image sensor and a second image sensor capture a first image and a second image including images of an object respectively. At a second time, the first image sensor and the second image sensor capture a third image and a fourth image including images of the object respectively. A coordinate calculation device calculates a first coordinate of the object at the first time according to the first image and the second image, and a second coordinate of the object at the second time according to the third image and the fourth image. A coordinate correction device calculates a displacement between the first time and the second time according to the first coordinate and the second coordinate, and corrects an output coordinate of the object at the second time according to the displacement.