As another method of noise/device property removal, for example, the noise/device property removal means 201 may average two or more observation acoustic signals obtained by performing an operation of observing the acoustic signal transmitted from the acoustic signal transmission means 101 by the acoustic signal observation means 102 a plurality of times. This method can reduce the influence of noise that occurs instantaneously.

Yet another method of noise/device property removal is a method of storing an average of observation acoustic signals observed from a plurality of persons beforehand and, upon authentication, subtracting the stored average from the observation acoustic signal observed from the user (first user) to be authenticated. The average of the observation acoustic signals observed from the plurality of persons is considered to contain common property not dependent on a specific person. Accordingly, noise and device property can be removed by subtracting such common property from the observation acoustic signal of the first user.