PURPOSE: To detect abnormal phenomena easily and surely such as the disconnection of strands, corrosion, etc., of an aerial cable by hammering the surface of the aerial cable and by making a comparison between the frequencies of sound caused by the hammering in each section moving the spot hit by the hammer in the longitudinal direction of the cable.

CONSTITUTION: A traveler 2 is hung on an aerial cable 1, downward which a microphone 4 is arranged. By radio control the traveler 2 is moved along the longitudinal direction of the aerial cable 1, while by driving a hammering device 3 the surface of the aerial cable 1 is hit with a ball 8 at the tip of a tap rod 7. At this moment, the hit sound is caught by the microphone 4 and is compared with sound data. In case the difference in frequencies is deviated from the reference value, it is judged to be abnormal. The corrosion, etc., inside the aerial cable 1 which cannot be discovered by visual check can be detected. No labor is imposed to workers by means of such an inspection robot.