We measure the wear of contact wire using the electric inspection car, and residual diameters are computed from a width of sliding surface of contact wire. As the principle, only the sliding surface reflect laser beam. Accordingly, we cannot get the actual width when other parts reflect. The reflections show noise data. In our study, we take notice that the side portions of contact wire exist both adjacent sides of sliding surface, never reflect the laser beam and show the shadows. Then, the shadows and the sliding surface point out the horizontal diameter of the contact wire. Therefore, we developed the system to eliminate the noise data owing to neglect the reflections of laser beam outside the contact wire.