To measure the deflection of a rotor using a frequency filter with higher accuracy than in manual measurements and to determine whether of not thermal distortion is acceptable.

A rotor 1 is put in the body 4 of a heating furnace and rotated, and a displacement measuring instrument 5 is used to measure radial displacement on at least eight circumferentially equally spaced positions on a predetermined portion of the outer peripheral surface of the rotor 1 along the direction of its central axis; the measurements are taken as electric signals, and to electric signals at the eight equally spaced positions are subjected to Fourier transform the obtain frequency components per unit rotation period of the rotor 1, after which they are subjected to signal processing using a frequency filter which allows only a predetermined range of frequencies including one rotation period of the rotor 1 to pass through; then, they are subjected to inverse Fourier transform to obtain smoothed signals, from the magnitude of the amplitude of which the terminal distortion of the rotor 1 is judged.