PURPOSE: To enable the accurate detection leakage without being influenced by the variation of background noise by comparing a threshold value selected according to a plant output with a signal given from acoustic sensors.

CONSTITUTION: Acoustic sensors 47a and 47b are pulled outside the shell 31 of humidity separation heaters 36a and 36b via wave conducting rods 45a and 45b and are installed at a low-temperature environment part. Thereby, the possibility of thermal deterioration is rather small. Spiral flexible parts 46a and 46b for heat expansion/contraction absorption are provided on the way of the wave conducting rods 45a and 45b, a failure due to the heat expansion/ contraction of a structured member is avoided. A leakage judging mechanism 53 judges the presence or absence of leakage by setting the threshold value according to a turbine load to reference. Thereby, the influence of the background noise varied depending upon the operation status of plant can be eliminated, and detection 50 of the tube leakage is performed accurately.