PURPOSE: To determine the position of the deterioration in insulation by sequentially moving the heating position of a cable, measuring the thermally stimulated current, and detecting the change in current.

CONSTITUTION: A sound cable and a deteriorated cable are used as the samples. A guard electrode 3 and a measuring electrode 4 of the cable 2 are connected to an ammeter 6 through lead wires 9 and 10. The cable 2 is moved at the speed of about 1m/h with moving rollers 5 through a cable heating device 1, and the current is measured. Meanwhile, a bias voltage Vb 8 is applied on the core conductor of the cable 2 for 10 minutes at ordinary temperature. Then, the cable is once grounded. A voltage (collecting bias Vc) 7 in the same polarity, which is smaller than Vb 8, is applied. The cable 1 is heated, and the thermally stimulated current flowing through the cable 2 is measured. As the value of the bias voltage Vb, 10kV/mm is sufficient. In actual detection, the sufficient detection can be performed when the detection is performed at the DC withstand voltage value of the cable to be detected. As the collecting bias Vc, about 1kV/mm, which does not give damage, is applied in order to improve the detecting sensitivity.