1. A personal authentication apparatus for certifying a user, comprising: a memory storing a template face-picture of the user therein, and a face-picture of a particular person who is categorized as of a special concern; an image pickup unit taking a face-picture of said user; a particular person comparing unit comparing said user's face-picture taken by said image pickup unit with the face-picture of the particular person, who is categorized as of the special concern, outputting as comparison result a degree of similarity therebetween, and deciding whether said degree of similarity is higher than a predetermined value; a personal picture acquiring unit for acquiring the template face-picture of the user from said memory; and an authentication unit, when said degree of similarity is higher than said predetermined value, deciding whether or not said user's face-picture taken by the image pick-up unit is identical with the template face-picture of the user by a first method, and when said degree to similarity is lower than said predetermined value, deciding whether or not said user's face-picture taken by the image pick-up unit is identical with the template face picture of the user by a second method, the second method being different from the first method.