To avoid leakage, critical damage and danger by detecting the position and extent of leakage quickly and surely using a simple system.

A duct or a hollow body 1 guides a liquid medium, e.g. fuel applied with a high jet pressure, brake liquid or cooling liquid. Wall part 2 of the hollow body 1 for guiding a liquid is applied with an extremely thin coating 3. The coating 3 is composed of a chemically or physically reactive substance which changes color upon absorbing a liquid of particular chemical composition, e.g. diesel fuel, cooling liquid or brake liquid containing alcohol, and thereby leakage can be detected. In some case, reaction of the coating 3 can be detected using UV-rays additionally. According to the arrangement, a trace of leakage can be detected quickly at a good timing and a countermeasure can be taken.