Detection of attachment problem of apparatus being worn by user

A personal authentication method comprising:

• authenticating an individual by using first information, the first information including at least one of an acoustic characteristic calculated from an acoustic signal and a feature amount extracted from the acoustic characteristic, the acoustic signal propagating through a head of a user and detected by an apparatus that is attached on the head of the user and that transmits and receives the acoustic signal;
• and
• detecting an attachment problem with respect to an attachment state of the apparatus, which is a state of the user wearing the apparatus on the head of the user, in response to the first information satisfying an attachment problem rule for detecting the attachment problem.

A non-transitory computer readable recording medium in which a personal authentication program is recorded, the personal authentication program causing a computer to execute:

• authenticating an individual by using first information, the first information including at least one of an acoustic characteristic calculated from an acoustic signal and a feature amount extracted from the acoustic characteristic, the acoustic signal propagating through a head of a user and detected by an apparatus that is attached on the head of the user and that transmits and receives the acoustic signal;
• and
• detecting an attachment problem with respect to an attachment state of the apparatus, which is a state of the user wearing the apparatus on the head of the user, in response to the first information satisfying an attachment problem rule for detecting the attachment problem.