PURPOSE: To ensure an assured detection of the signal produced by occurrence of the crack of the turbine rotor, by detecting the elastic wave discharge signal generated from the turbine rotor via the wide-band AE element and then comparing the frequency distribution signal of the elastic wave discharge signal with the freuency distribution signal of the normal mode in the beginning of operation.

CONSTITUTION: The following units are provided: the wide-band AE element 1 incorporating a radio trnasmitter attached to the turbine rotor; the receiver 2 which receives the signal from the element 1; the A/D converter 4 which converts the received electric signal into the digital signal; and the digital memory 5 which stores the output signal sent from the converter 4. In addition, the frequency analyzer 6 is provided to give a simultaneous frequency analysis to the signals sent from both the converter 4 and the memory 5, also with the comparator 7 which gives a comparison between the two signals delivered from the analyzer 6 and then decides level to the set value. And in case the deviation between the two signals C and B delivered from the analyzer 6 becomes larger than the set value, the alarming circuit 8 is actuated.