PURPOSE: To measure the length of a composite cable accurately, by performing computation based on a time, during which light is inputted to one end of an optical fiber, reflected by the other end, and returned, and the propagation speed of light.

CONSTITUTION: An overhead earth wire 1 forms a composite cable along which an optical fiber 2 is provided in the longitudinal direction. Light from a light emitting part 5 is fed to one end of the optical fiber 2 through an optical fiber transmission path 4. The light is reflected by a reflecting mirror 8 at the other end of the optical fiber 2 and outputted from the other end through the optical fiber 2 again. The reflected light is fed to a light receiving device 6 through an optical fiber transmission path 4'. The time, during which the light is inputted to the optical fiber 2 and returned, is measured. Based on the measured value and the propagation speed of the light in the optical cable 2, the total length of the optical fiber 2 is computed. Thus the length of the composite cable can be accurately measured.