Abstract: A user authentication apparatus for use in controlling access to a system inputs an owner's login name and password and then extracts the owner's timing vectors from keystroke characteristics with which the owner repeatedly types the owner's password to thereby form a training set. A neural network is trained by using each of the owner's timing vectors in the training set as an input. Thereafter, when a user inputs the owner's login name and password, it is checked if the user's password is identical to the owner's password. The user's timing vector is extracted from a keystroke characteristic to type the user's password if the checked result is affirmative, and the user is prohibited from accessing the system if otherwise. The user's timing vector is applied to the trained neural network as an input and a difference between the input and an output of the neural network is compared with a predetermined threshold.