PURPOSE: To detect an abnormality of the rotary shaft without stopping the shaft by deciding the magnitude of torsional oscillation dampling rate obtained by measuring an unevenness of the rotation.

CONSTITUTION: A gear-shaped disc 5 is provided to the end of rotary shat, an electromagnetic pickup 6 for detecting the rotation pulse is placed beside the disc. The uneven-rotation detecting unit consists of the disc 5, pickup 6 and unevenrotation meter 8. A damping oscillation wave form from the filter 9 which takes only a specific frequency component of the output signal of the meter 8 is computed by the arithmetic unit 10 to obtain the torsional oscillation damping rate. The stored value, which is obtained such that a torsional oscillation damping rate of the normal state of shaft 4 is multiplied by certain constant, and an output of the arithmetic unit 10 are compared with each other by the comparator 11 to decide an abnormality of the shaft 4, an alarm is delivered if the shaft is unusual. By this way, an abnormality of the rotary shaft can be detected as the shaft is turned.