To provide a method and an apparatus wherein an operation to specify an accident point existing in a complicated branch cable line such as a cable line branched further from a branch line is performed easily and with satisfactory efficiency.

A high voltage is applied to a main line 1 and a first branch line 16 to a fifth branch line 20 by a power-supply device 11, and it is discharged in the accident point β. A surge current generated as a result of its discharge is detected by sensors (1) to (7). Its arrival time and its detection level are found. On the basis of the arrival time of the sensor (4) whose detection level is highest and that of the sensor (3) whose detection level is second highest, a calculation to find the accident point β is performed.