An acceleration sensor is attached to a rack housing. A FFT processing unit converts a signal output from the acceleration sensor, which is a time-domain signal, into a frequency-domain signal. An inverse input vibration component extraction unit extracts a signal, of which the frequency (f) falls within a predetermined range (fL≦f≦fH (fH>fL)) and of which the power density (ρ) falls within a predetermined range (ρL≦ρ≦ρH (ρH>ρL)), from the frequency-domain signal obtained by the FFT processing unit. An IFFT processing unit converts the frequency-domain signal extracted by the inverse input vibration component extraction unit into a time-domain signal (estimated inverse input vibration).