PURPOSE: To monitor the fatigue of a shaft thereby to prevent the shaft from being broken by monitoring the variation in the torsional intrinsic frequency of the shaft system in terms of reduction of the torsional spring constant of the shaft due to the fatigue and cracking in the shaft.

CONSTITUTION: A pulse generator 10 generates pulses of such frequency as is proportional to the r.p.m. of the shaft 4, and a rotational irregularity meter 11 meters the fluctuations in the pulse frequencies to detect the torsional vibrations of the shaft 4. When there arise disturbances in torque such as abrupt short-circuiting, the waveforms of the meter 11 are analyzed by means of a frequency analyzer 12. Then, as shown in solid lines, a peak is found at the torsional intrisic frequency f1 of the shaft system. Here, the intrinsic frequency is deduced in advance. When the shaft 4 is subjected to fatique or cracking due to a sudden accident, the cross- sectional area of the shaft is reduced so that the torsional spring constant is accordingly decreased to shift the intrinsic frequency f1 downward to f2. Thus, it is judged by a discriminator 14 whether the frequency f1 is shifted to f2 or not. If this shift takes place, the result is displayed in a device 15, and an alrm is generated.