A control unit makes a screen of a display unit display a plurality of pattern display areas to which a predetermined respective plurality of patterns are uniquely assigned at random as a pattern random array in the same layout as that of a plurality of input keys capable of inputting numerical values. The control unit determines a shortest path to go through a sequence of patterns serving as a password of a user in the pattern random array, and replaces a sequence of all patterns existing on the shortest path with a sequence of codes assigned to the respective plurality of input buttons according to a correspondence relationship between the plurality of input buttons and the plurality of pattern display areas to generate a one-time password. The control unit then compares the one-time password with a sequence of codes input by the user by using an input unit to perform authentication.