1. A user authentication apparatus for use in controlling access to a system comprising: an input means for inputting an owner's login name and password, and a user's password; a password matching means for storing the owner's login name and the owner's password and checking whether the user's password is identical to the owner's password or not, to thereby generate a first indication signal; means for extracting the owner's timing vectors from keystroke characteristics with which the owner repeatedly types the owner's password to thereby form a training set and, in response to the first indication signal from the password matching means extracting a user's timing vector from a keystroke characteristic with which the user types the user's password; a neural network for being trained by using the training set and generating an output by using the user's timing vector as an input; means for comparing the difference between the user's timing vector that is inputted to the neural network and the corresponding output of the neural network with a predetermined threshold to thereby generate a second indication signal; and means for controlling access of the user to the system in response to the first and the second indication signals, wherein the password matching means generates the first indication signal of level 1 if the user's password is identical to the owner s password and level 2 if otherwise.