Undulating wear of overhead rigid conductor line causes serious trouble, such as extreme wear of contact line and contact strips of pantograph. In order to analyze the mechanism of wear and propose countermeasures against this problem, we have carried out investigation of wear characteristics of real lines and excitation tests of pantograph. Based on the results of these tests, this paper reports the condition to easily generate undulating wear, discusses the mechanism of wear process and proposes some countermeasures to prevent undulating wear.The unevenness measuring device used in this study consists of two pairs of laser sensors to reduce measurement errors due to the vertical vibration of measurement vehicle. The unevenness of sliding surface of rigid conduction line was measured accurately and continuously with this device. The measured undulating wear was characterized as follows. The sliding surface of conductive rail is roughened with serious arc spots caused by contact loss of pantograph; the maximum amplitude of the unevenness (peak-to-peak) is about 0.4 mm (Fig. 1); the wavelength is about 12 cm, which is equivalent to half the distance between two contact strips of pantograph; and undulating wear tends to occur in the section where trains are in powering at about 45 km/h (Fig. 2) and the deviation of rigid contact line is larger than 50 mm.