As undulating wear on an overhead rigid conductor line grows up, arcs due to contact loss occur frequently between the conductor line and a pantograph. The arcs cause extreme wear of the conductor line and contact strips of the pantograph. However, mechanism of the undulating wear formation has not been clarified. In order to clarify this, the authors investigated state of sliding surface of overhead rigid conductor lines on commercial lines and dynamic characteristics of a pantograph. These results indicate that the dynamic characteristics of the pantograph and the distance between pantograph heads play a causal role in the undulating wear formation. Periodic unevenness is formed by mechanical wear due to the dynamic characteristics of a pantograph, in particular anti-resonance phenomenon of the pantograph affects this process significantly. Once the undulating wear amplitude grows to the extent that the pantograph cannot keep contact with the conductor line, arcs due to contact loss frequently occur at concave portion of undulating conductor causing extreme undulating wear. The wavelength of the extreme undulating wear is related to the interval of the pantograph heads.