PURPOSE: To simplify the measurement of the degree of twist vibration at a desired area, by analyzing the twist vibration waveform of a rotary shaft for each natural vibration mode and adding the degrees of natural vibrations to multiply the added degree by a prescribed coefficient.

CONSTITUTION: A twist vibration waveform ψ0 of a rotary shaft 4 is detected by a twist angle detector 6 and analyzed into waveforms ψ1Wψn through BPFs 71 and 7n. Each waveform is sampled by a sampling device 8, and each peak value of the sine wave-shaped attenuated answer wave is stored along with the time when the answer wave is produced. An arithmetic device 9 obtains the degree of approximate natural vibration of each mode from the stored peak value and its time of production and adds all degrees to multiply the proper coefficient that is previously obtained at the area where the twist vibration degree of the shaft 4 is to be obtained. Thus the degree of twist vibration is calculated. The result of this calculation is fed to an output device 10.