PURPOSE: To attain safety management by comparing the stress calculated by inputting a signal outputted from a strain gauge sticked to the surface of a diaphragm with the previously set up allowable stress.

CONSTITUTION: A strain signal detected by a strain gauge 27 is superposed to a transmitting wave by an oscillator 16 and transmitted to a receiver 17. The strain signal is taken out from the receiver 17, inputted to an operator 18 to convert the strain into the value of stress and then the converted value is inputted to a comparator 19. The comparator 19 compares a previously set up allowable stress value with the stress value inputted from the operator 18 and, when a discriminator 20 discriminates that the inputted stress value exceeds the allowable stress, a load reducing instruction is sent to an operation controller 21. When another instruction is not received from the discriminator 20, the operation controller 21 controls the output of a generator plant concerned.