The earphone 14 corresponds to the above-mentioned acoustic signal transmission means 101. The microphone 15 corresponds to the above-mentioned acoustic signal observation means 102. The microphone 15 and the earphone 14 are desirably integrated so that their relative positional relationship is unchanged, as depicted in FIG. 2. The present invention is, however, not limited to such, as long as their relative positional relationship is not changed significantly. In FIG. 2, a microphone-integrated earphone that is inserted into the entrance of the ear canal is depicted as an example of the acoustic signal transmission means 101 and the acoustic signal observation means 102. Alternatively, the acoustic signal transmission means 101 and the acoustic signal observation means 102 may be realized by a pinna-covering headphone provided with a microphone (pinna type microphone-integrated earphone). The acoustic signal transmission means 101 and the acoustic signal observation means 102 may be realized by a telephone having a microphone in its receiver portion. In such cases, separate functions may be performed on the right and left sides, e.g. observing, with a microphone located at the entrance of the right ear canal, an acoustic signal transmitted from an earphone located at the entrance of the left ear canal, or vice versa.

The acoustic property calculation means 103, the feature extraction means 104, and the user identification means 105 are each realized by a CPU and memory (both not depicted) included in the PC 11, where the CPU operates according to a program. The feature value storage means 106 is realized by a storage medium (not depicted) such as a hard disk included in the PC 11.