PURPOSE: To increase a detecting capacity by making a smoothing value being not influenced by the fluctuation in the amplitude of a measuring signal.

CONSTITUTION: An adaptive type filter 8 calculates the optimum approximate measuring signal of the measuring signal which is stored in a data buffer 4 and outputs it as mean value. The amplitude of the measuring signal is adjusted so that the mean value extending for several tens seconds of the amplitude of the measuring signal becomes constant by the automatic gain adjuster 3 which is added before the adaptive type filter 8 and the adaptive type filter 8 can make the pass band property of a low-band filter and the amplitude of the smoothing value not influenced by the size of the amplitude of the measuring signal. Therefore the differentiation filter 9 provided behind the adaptive type filter 8 can output the differentiation value without being influenced by the size of the amplitude of the measuring signal as well, so double decision is performed by combining the feature extracting signal extracted by the conventional Haar filter 10. The identifying capacity of the signal can thus be increased without being influenced by the size of the amplitude of the measuring signal.