With such a structure, personal authentication can be performed without a special action by the user for authentication. Personal authentication that lessens the psychological and/or physical burden on the user to be authenticated can thus be realized.

Although not depicted, the personal authentication device may further include feature extraction means for extracting a feature value relating to the user from the acoustic property. In this case, the user identification means may identify the user, based on the feature value extracted by the feature extraction means and relating to the user.

The foregoing exemplary embodiments can be described as, but not limited to, the following supplementary notes.

(Supplementary note 1) A personal authentication device including: acoustic signal transmission means for transmitting a first acoustic signal to a part of a head of a user; acoustic signal observation means for observing a second acoustic signal which is an acoustic signal after the first acoustic signal propagates through the part of the head; acoustic property calculation means for calculating an acoustic property from the first acoustic signal and the second acoustic signal; and user identification means for identifying the user, based on the acoustic property or a feature value extracted from the acoustic property and relating to the user.