PURPOSE: To detect speedily leakage from a lined vessel and to prevent a leakage signal from outputting due to dewing, by providing a leakage detector which outputs the leakage signal by the contact of fluid and a dry air supplying device which supplies dry air where the leakage detector is installed.

CONSTITUTION: Leaking water spreads between a lining plate 4 and a concrete body 2 and sticks immediately to the leakage detector 9 at a cracking installation part, and consequently the leakage detector 9 varies in resonance impedance greatly. A leakage monitoring device 12 detects the variation in resonance impedance of the leakage detector 9 through a connection line 11 to output a warning signal to, for example, a warning device. Then, an opening and closing valve 17 inserted into a vent pipe 14 is opened and a pressure reducing valve 16 is regulated to supply irreducible dry air which never causes dewing from a cylinder 15 to a conduit groove 5 arranged at the leakage detector 9 and a recessed part 8 formed in the concrete body 2 at all times, so no water due to dewing sticks to the leakage detector 9.