To realize a structure capable of facilitating the mounting work to a cable 1 and of sufficiently securing the determination accuracy of a failure point 2 at a low cost.

Only one clock (counter circuit 11) is used for measuring a time that a surge current generated in conjunction with the discharge at the failure point 2 reaches both ends of the cable 1. By virtue of this structure, inconvenience in the case of using two clocks can be eliminated and the purpose can be accomplished.