An biological-information authentication device includes, a biological-information reading section configured to read biological information; a comparing section configured to compare biological information read by the biological-information reading section with registered biological information to determine whether they match; a registration section configured to register biological information; a biological-information-input-operation extraction section configured to extract input operation data indicating an input operation of biological information read by the biological-information reading section. The device further includes an approval-input-operation determination section configured to determine whether the input operation data of the biological information extracted by the input-operation extraction section matches approval input operation data indicating an input operation for approval that is stored in advance when the comparing section determines that the biological information is unregistered biological information and that biological information input next to the unregistered biological information matches the registered biological information of an administrator having approval authority.