In the case where the acoustic signal transmission means 101 transmits the same acoustic signal, the acoustic signal observation means 102 observes the same acoustic signal as long as the user and his or her position are unchanged. In this case, there is a possibility that a malicious person eavesdrops the observation acoustic signal and succeeds in impersonation by some means. To prevent such an act, it is preferable to change the transmission acoustic signal in the case where the state in which the user and his or her position are unchanged continues. In this way, even if the observation acoustic signal is eavesdropped for impersonation, in the case where a process of comparison with the transmission acoustic signal results in at least a predetermined difference in signal element between the two signals, the user to be authenticated can be rejected as an impostor.

Moreover, by separating the signal path until the acoustic signal transmission means 101 transmits the transmission acoustic signal and the signal path until the acoustic signal observation means 102 receives the transmission acoustic signal in the personal authentication device, the risk of simultaneous eavesdropping of two acoustic signals (transmission acoustic signal and observation acoustic signal) can be avoided.

Exemplary Embodiment 6 of the present invention is described below. The structure of this exemplary embodiment may be the same as that of Exemplary Embodiment 1, except that the acoustic signal transmission means 101 in this exemplary embodiment uses musical sound as a transmission acoustic signal. This further lessens the psychological/physical burden on the user.