An example of the method of removing noise and device property from the acoustic property is a method of subtracting common acoustic property obtained from observation acoustic signals observed at two locations of the head of the user. Another example is a method of averaging acoustic property obtained from observation acoustic signals resulting from observation performed a plurality of times. Yet another example is a method of subtracting acoustic property obtained from an average of observation acoustic signals observed from a plurality of persons, from the acoustic property obtained from the observation acoustic signal observed from the user to be authenticated.

The method of removing noise and device property from the feature value may be the same as the method of removing noise and device property from the acoustic property, i.e. the method of subtraction or averaging (with “acoustic property” being replaced with “feature value”), or a method of measuring the standard deviation of the acoustic property and performing division.

Each of the noise/device property removal means 201, the noise/device property removal means 202, and the noise/device property removal means 203 may remove device property by subtracting, from the observation acoustic signal (or acoustic property or feature value obtained from the observation acoustic signal), an acoustic signal (or acoustic property or feature value obtained from the acoustic signal) obtained by propagation through only the earphone and the microphone without involving the head.

Exemplary Embodiment 2 is the same as Exemplary Embodiment 1 in other respects.