At a first time, an image sensor captures a first image including images of an object. At a second time, the image sensor captures a second image including images of the object. A coordinate calculation device calculates a first coordinate of the object at the first time according to the first image, and a second coordinate of the object at the second time according to the second 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.