The present invention is to easily and reliably measure the wear over the full section of a trolley wire even in the daytime without being influenced by structures around the trolley wire. The test car (1) has a line sensor (5) arranged along the direction of the scanning line perpendicular to the direction in which a trolley wire is laid and an illuminator lamp (6) and images the pantograph contact surface of the trolley wire while thee test car is traveling. The measuring computer (7) stores the image in a recorder (8). A line sensor image is created by arranging in time series the luminance signal of the scanning line obtained by the line sensor, a binary line sensor image is created from the line sensor image, the sky parts are removed from the binary line sensor image, the edge of the worn part of the trolley wire is detected, and the width of the worn part of the trolley wire is determined from the full width of the trolley wire with respect to the edge data and the height of the trolley wire viewed from the line sensor.