PURPOSE: To permit searching for the direction and position of insulation defects on route during power transmission by connecting a heat accumulator between more than one position of metal shield layers and the ground, then by removing the earthing of the shield layer, and by connecting a microcurrent measuring means between the shield layer and the ground.

CONSTITUTION: A large capacity of the heat accumulators 10 and 10' are connected to between the metal shield layer 2 and the ground at the both ends of cable, and the anticorrosive layer 2 is removed at two positions at a predetermined interval in the longitudinal direction of cable to expose the shield layer 2. Then, the terminals a and b are attached to one of the shield layer 2, the terminals c and d are attached to the other one of the shield layer 2, the measuring power source 4 is connected through the switch 5 to between the terminals b and c, and the galvanometers 6 and 6' are connected to between each of the terminals a and d and the ground, by means of which the direction of insulation defective resistor 7 can be detected. Thus, the direction of insulation defects can be safely detected even during power transmission.