(Supplementary note 2) The personal authentication device according to supplementary note 1, including storage means for storing beforehand, for at least one person, an acoustic property at a time of propagation of an acoustic signal through a predetermined part of a head of the person or a feature value extracted from the acoustic property, wherein the user identification means identifies the user, by comparing the acoustic property calculated from the second acoustic signal with the acoustic property stored in the storage means or comparing the feature value extracted from the acoustic property calculated from the second acoustic signal with the feature value stored in the storage means.

(Supplementary note 3) The personal authentication device according to supplementary note 1 or 2, wherein the part of the head which is a propagation path of the first acoustic signal includes at least one ear canal and/or pinna.

(Supplementary note 4) The personal authentication device according to any one of supplementary notes 1 to 3, wherein the acoustic signal transmission means is realized by an earphone, a headphone, or a receiver portion of a telephone, and wherein the acoustic signal observation means is realized by a microphone mounted on the earphone, the headphone, or the receiver portion of the telephone.

(Supplementary note 5) The personal authentication device according to any one of supplementary notes 1 to 4, including removal means for removing ambient noise and/or a device property from the second acoustic signal, based on acoustic signals observed at two locations of the head of the user, acoustic signals observed from the same user a plurality of times, or acoustic signals observed from a plurality of persons.