PURPOSE: To estimate whether change has occurred in a body under test of or not highly accurately by detecting the statistic difference between the statistic variate based on the remainder signal obtained by acting a reverse filter on a plurality of the first time-series signals and the statistic remainder obtained from the second time-series signals by the same way. CONSTITUTION: Background noises are collected by an ultrasonic microphone 10. The signal in the initial time zone of the background noises is inputted to a reverse-filter forming part 11. The background noises are continuously collected with the microphone 10. The collected background noise signal is inputted to a reverse-filter acting part 12. The acting part 12 makes the reverse filtering formed in the forming part 11 act on the inputted background noise signal, and each remainder signal is obtained and inputted into a variate operating part 13. The operating part 13 operates the average power spectrum of each remainder signal. The maximum value and the minimum value of the smooth power spectrum between the specified frequencies are extracted. The difference between the maximum value and the minimum value as the statistic variate is obtained.