Abstract: A personal authentication device includes: acoustic signal transmission means 701 for transmitting a first acoustic signal to a part of a head of a user; acoustic signal observation means 702 for observing a second acoustic signal which is an acoustic signal after the first acoustic signal propagates through the part of the head; acoustic property calculation means 703 for calculating an acoustic property from the first acoustic signal and the second acoustic signal; and user identification means 704 for identifying the user, based on the acoustic property or a feature value extracted from the acoustic property and relating to the user.