Exemplary Embodiment 4 of the present invention is described below. FIG. 10 is a block diagram depicting an example of the structure of a personal authentication device in Exemplary Embodiment 4 of the present invention. The personal authentication device depicted in FIG. 10 has a structure in which noise signal estimation means 401 and transmission acoustic signal selection means 402 are added to the structure of Exemplary Embodiment 1 depicted in FIG. 1.

The noise signal estimation means 401 estimates a noise signal from an observation acoustic signal. For example, the noise signal estimation means 401 estimates a noise signal, from an observation acoustic signal observed at or near a part of the head of the user in a state where an acoustic signal for authentication is not transmitted.

The transmission acoustic signal selection means 402 selects an acoustic signal of a frequency band different from the frequency band of the estimated noise signal, as a transmission acoustic signal.

FIG. 11 is a flowchart depicting an example of the operation of the personal authentication device in this exemplary embodiment. The same operations as those in Exemplary Embodiment 1 are given the same reference signs, and their description is omitted. In the example depicted in FIG. 11, the personal authentication device observes an acoustic signal using the acoustic signal observation means 102, before transmitting an acoustic signal to the part of the head of the user (step S201).

Subsequently, the noise signal estimation means 401 estimates a noise signal using the observed acoustic signal (step S202).