For the maintenance of overhead contact line, it is important to know the exact state of contact wire wear in electric railways. A new type of measuring apparatus has been developed, which is sufficiently small to be installed on the roof of commercial car. The apparatus has an optical system composed of sodium vapor lamps and CCD line sensor cameras with optical filters. This apparatus detects the width of wear surface of wire, and calculates the residual height from the width. The measuring frequency of the cameras is 1 kHz. The new apparatus has been tested in a laboratory and in the field by using an inspection car. The results show that it can be used at the speed of 100 km/h in daytime at a standard deviation of measuring error of 0.3 mm.