1. A face authentication system, comprising: a photographing unit configured to detect a face image of an authentication target person by using an auto focus lens equipped therewith; and a computer system coupled to the photographing unit and defining a distance extraction unit configured to extract a distance from the photographing unit to a face of the authentication target person based on a position of the auto focus lens when detecting the face image of the authentication target person; a feature extraction unit configured to extract feature points of the face image of the authentication target person detected by the photographing unit and distances between the extracted feature points; an analysis unit configured to perform an analysis by comparing the distance to the face of the authentication target person extracted by the distance extraction unit with pre-registered distance information of the face of the authentication target person, and comparing the feature points and the distances between the feature points extracted by the feature extraction unit with pre-registered face image information of the authentication target person; and an authentication unit configured to authenticate the authentication target person based on a result of the analysis from the analysis unit, wherein the face image information includes feature point information on feature points of the face of the authentication target person, and distance information on distances between the feature points of the face of the authentication target person.