In this paper we propose a method for carrying out personal authentication by recording a photo of a computer user's face, recognizing the shape of their lips and front teeth, and performing image matching. Using this method, personal authentication is carried out by comparing the previous and current shape of a user's lips and front teeth when they log on to a computer. This method has a number of merits. As teeth are normally hidden behind the user's lips, user authentication is not carried out automatically as is the case with facial recognition. This means that it is possible for the user to decide for themselves whether or not to carry out user authentication. As it is not possible for a user to be recognized simply by their face being recorded on camera, their privacy is protected. Additionally, as the shape of lips and front teeth vary between individuals, there is a high probability that a user can be correctly identified. Furthermore, there is a low probability of significant changes to the shape of adult teeth over a period of months or years unless major dental work is carried out, so this method is potentially suited to personal authentication. Another merit of this method is the fact that it is possible to perform user authentication using an inexpensive webcam. In this paper, we will detail experiments using this method to perform personal authentication as well as the subsequent results.