PURPOSE: To detect the generation of friction surely and automatically, by extracting oscillation signal components synchronized with the rotation speed of a rotating material and comparing the level of oscillation signal components with a preliminarily set alarm level.

CONSTITUTION: When a steam turbine is started, the oscillation of the whole of the turbine including oscillation components of a rotor is detected as an electric signal by an oscillation detector 1 and is amplified in an amplifier 2 and is detected in a detector 3 and is applied to an analyzer 6. A signal of the rotation number of the steam turbine detected by a speed detector 4 is converted to a voltage signal by a signal converter 5 and is applied to the analyzer 6, and oscillation signal components synchronized with the rotation frequency at this time are extracted. A signal corresponding to the peak value of them is applied to a comparator 8; and if the output level exceeds the alarm level of an alarm setter 7, an alarm is generated by an alarm relay.