A personal authentication device comprising: a storage means; a transmission means configured for transmitting a first acoustic signal toward a cavity of a head of a user from an entrance of the cavity, the cavity being an ear canal; an observation means configured for observing a second acoustic signal that is an acoustic signal after the first acoustic signal propagates through the cavity of the head of the user; a calculation means configured for calculating acoustic characteristics from the first acoustic signal and the second acoustic signal; an extraction means configured for extracting a feature amount related to the user from the acoustic characteristics; and a storage control means configured, when a service is started, to store the extracted feature amount in the storage means, as a registered feature amount, wherein when the service is being executed, the transmission means, the observation means, the calculation means, and the extraction means are configured to repeat their respective operations, thereby extracting anew a feature amount, the personal authentication device further comprises: an identification means configured for identifying the user by i) collating the registered feature amount in the storage means with the feature amount extracted anew, and ii) determining whether the user is identical by determining whether the registered feature amount in the storage means and the feature amount extracted anew coincide with each other, wherein when the identification means identifies the user as being identical, the transmission means is configured to transmit the first acoustic signal after a predetermined time has passed, and the storage control means is further configured to delete the registered feature amount from the storage means when the service is stopped. a storage means; a transmission means configured for transmitting a first acoustic signal toward a cavity of a head of a user from an entrance of the cavity, the cavity being an ear canal; an observation means configured for observing a second acoustic signal that is an acoustic signal after the first acoustic signal propagates through the cavity of the head of the user; a calculation means configured for calculating acoustic characteristics from the first acoustic signal and the second acoustic signal; an extraction means configured for extracting a feature amount related to the user from the acoustic characteristics; and a storage control means configured, when a service is started, to store the extracted feature amount in the storage means, as a registered feature amount, wherein when the service is being executed, the transmission means, the observation means, the calculation means, and the extraction means are configured to repeat their respective operations, thereby extracting anew a feature amount, the personal authentication device further comprises: an identification means configured for identifying the user by i) collating the registered feature amount in the storage means with the feature amount extracted anew, and ii) determining whether the user is identical by determining whether the registered feature amount in the storage means and the feature amount extracted anew coincide with each other, wherein when the identification means identifies the user as being identical, the transmission means is configured to transmit the first acoustic signal after a predetermined time has passed, and the storage control means is further configured to delete the registered feature amount from the storage means when the service is stopped. an identification means configured for identifying the user by i) collating the registered feature amount in the storage means with the feature amount extracted anew, and ii) determining whether the user is identical by determining whether the registered feature amount in the storage means and the feature amount extracted anew coincide with each other, wherein when the identification means identifies the user as being identical, the transmission means is configured to transmit the first acoustic signal after a predetermined time has passed, and the storage control means is further configured to delete the registered feature amount from the storage means when the service is stopped.