Provided is a personal authentication device capable of simply securing security with little psychological and physical burden of a user to be authenticated. Personal authentication device includes: transmission unit that transmits a first acoustic signal to a user's head; observation unit that observes a second acoustic signal after the first acoustic signal propagation; calculation unit that calculates acoustic characteristics from the first and the second acoustic signal; extraction unit that extracts a feature amount related to a user from the acoustic characteristics; storage control unit that registers the feature amount in the storage unit; identification unit that identifies the user by collating the first feature amount with a second feature amount; and storage unit stores the first feature amount, wherein while identification unit identifies the user as being identical, transmission unit transmits the first acoustic signal every predetermined interval.