PURPOSE: To achieve a higher reliability, by placing a sample cable in an argon gas atmosphere to apply a test voltage between a core thereof and the enclosure.

CONSTITUTION: A sample cable 1 is placed into a closed container 14 and one core 2 of the cable 1 is connected to a positive terminal of a power source 10 through ammeter 11. On the other hand, a negative terminal of the power source 10 is connected to an earth net 15 of the container 14. Then, first, valves of cocks 16 and 18 are closed to make the inside of the container 14 vacuum with a vacuum pump 19 and a cock 17 is closed. Then, the cock 18 is opened to fill the container 14 with argon gas. Here, when a pinhole or the like exists in the cable 1, the argon gas passes through the pinhole to creep about the core 2. Here, the cock 18 is closed while the cock 17 is opened, the power source 10 is turned ON making the inside of the container 14 vacuum again to apply about 5,000V. If any pinhole exists, a spark current flows between the core 2 and the earth net 15 through the argon gas leaked from the pinhole, thereby enabling detection of the pinhole.