Subsequently, the transmission acoustic signal selection means 402 selects a transmission acoustic signal, based on the estimated noise signal (step S203). The subsequent operations may be the same as those in Exemplary Embodiment 1.

According to this exemplary embodiment, the influence of noise such as ambient environmental noise can be avoided.

Exemplary Embodiment 5 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 changes the transmission acoustic signal regularly or irregularly. An example of the irregular change is a method of, in the case of performing authentication randomly, changing the transmission acoustic signal each time authentication is performed. Another example is a method of, in the case of changing the number of times authentication is performed or the frequency at which authentication is performed depending on authentication reliability, changing the transmission acoustic signal each time authentication is performed. Yet another example is a method of, in the case of performing authentication each time the user makes some kind of service request, changing the transmission acoustic signal each time authentication is performed. Yet another example is a method of, in the case of performing authentication at timing of an interval of a service such as a change of music, changing the transmission acoustic signal each time authentication is performed.