In this exemplary embodiment, the acoustic property can be acquired in a relatively short time of about one second. This enables personal authentication to be performed continuously before and after and during service provision. By continuously performing personal authentication using the personal authentication method in this exemplary embodiment, an unauthorized act such as impersonation by another person after authentication can be detected, as compared with the case of performing authentication only the first time or only immediately before some service is provided.

Exemplary Embodiment 2 of the present invention is described below. FIG. 6 is a block diagram depicting an example of the structure of a personal authentication device in Exemplary Embodiment 2 of the present invention. The personal authentication device depicted in FIG. 6 differs from the structure of Exemplary Embodiment 1 depicted in FIG. 1 in that noise/device property removal means 201 is further included.

The observation acoustic signal contains not only acoustic property derived from the user's biometrics, but also various noise such as ambient environmental noise, cardiac sound, respiratory sound, utterance, joint sound, and vibration. Besides, the acoustic signal transmission means 101 and the acoustic signal observation means 102 themselves have acoustic property, which may appear as device property that represents an individual device difference.