PURPOSE: To improve decision precision by deciding leakage when either of thee increase pattern of the cumulative generation frequency of a burst type AE and the signal level of the specific frequency band of an AE original signal exceeds a set value and the plant control signal of a boiler decreases below a set value.

CONSTITUTION: The burst type AE is detected 19 in the AE original signal obtained from the output of an AE sensor 5 fitted to a heat conduction pipe 2. When the increase pattern of the cumulative generation frequency of this burst type AE increases above the set increase pattern, a comparing part 24 outputs the 1st detection signal. Further, a comparing part 29 outputs the 2nd detection signal when the signal level of the specific frequency band of the AE original signal increase the set value. Then, a decision on a leak is made only when the plant control signal from the boiler decreases below the set value after at least either of the 1st and the 2nd signals is outputted. Consequently, the precision of the leak decision making is improved.