The measuring way of verticality of elevator's guide way is studied in this paper and a measuring apparatus is made too. The apparatus regards the laser beam, emanating from the laser device, which can emit the perpendicular laser to the earth, as the measuring norm. The facula on the autodyne, which clings to the guide way during the measuring, is sampled to computer with a CCD camera and an image acquisition card. By computer image processing, the measuring apparatus can identify the displacement of the facula center. By this way it determines the verticality error of the guide way at the measuring position, which can be recorded automatically at the same time. For the facula after threshold segmentation, the apparatus can recognize it automatically. If it is circular, its center can be worked out by centroidal equation; if it is cirque, after edge detecting, its center can be figured out with Hough transform. For the Hough transform of circle, the dimension of its parameter space can be brought down from 3 to 2 with the derivative of the circle equation and thus its calculating quantity can be reduced greatly. The measuring way is proved by testing that it had improved more greatly and is more accurate, faster and simpler than ever before.