Water tree is a significant degradation mode of power cable with polymeric insulation. As those installed long time ago is supposed to be degraded with water trees, their diagnosing technique is being required for effective replacement. Especially locating the degraded region along the length would be helpful, since only a part of the cable line is usually degraded. As inside of the water tree is filled with trap sites, it is polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage is determined by ambient temperature, applying a “depolarizing voltage” with the opposite polarity can accelerate the process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree through looking at the time-resolved pulse response. This would lead to a diagnosing method with spatial resolution. In order to retain 100 m of spatial resolution, the response should be as sharp as 1 µs. As a preliminary study, a coaxial communication cable was employed as the specimen. An experiment using 405 m-long cable, with 5 m of degraded length in the middle, was performed. Two cables, one of which included degradation and the other did not, were oppositely coupled with a current transformer in order to cancel a rush current when the pulse voltage was applied. A sharp current response was observed, suggesting that a rapid depolarization took place. No such response was seen when the cable specimen was not applied with a poling bias voltage. The spatial resolution was equivalent to 50 m. We concluded that the technique is quite feasible.