PROBLEM TO BE SOLVED: To accurately and continuously measure the ruggedness of a long length body without being affected by the vertical vibration of a measurement base.
SOLUTION: A non-contact displacement sensor 1a detects trolley line displacement at a measurement spot (x) and outputs a first displacement signal and a second non-contact displacement sensor 1b detects the trolley line displacement at the measurement spot (x+Δx) and outputs a second displacement signal. The displacement signals are respectively passed through amplifiers 2a and 2b, A/D converters 3a and 3b and low-pass filters 4a and 4b and inputted to an inclination computing element 5. The inclination computing element 5 obtains a displacement difference Δy from the second displacement signal and the first displacement sensor and calculates an inclination signal Δy/Δx from that and a sensor head attachment interval Δx further. An integrator 6 executes correction by the ruggedness wavelength λ of the long length body to the inclination signal Δy/Δx, performs integration and outputs the ruggedness signal of the long length body to an output device 8.