The unevenness of contact lines is one of the factors that greatly influence the dynamic characteristics of current collection systems. In order to analyze the influence of the contact line unevenness on the contact force fluctuations between contact lines and pantographs, an instrument was devised to measure the unevenness of contact lines both accurately and continuously. The instrument consists of two pairs of laser sensors to reduce measurement errors caused by the vertical vibration of the measurement vehicle. This paper describes a way of measuring contact line unevenness, reports on some results obtained from measuring actual lines, and proposes a method to evaluate the conditions of overhead contact lines.