An authentication device comprising: a storage unit for storing shared information including password and shared rule that are preset for each user; a random number sequence generation unit for generating a random number sequence pattern; a calculation unit for generating a key code based on the shared rules, using provisional numerals selected from the random number sequence pattern and secret numerals constituting passwords; a transceiver for receiving identification information of a user to be authenticated from an input terminal, transmitting the random number sequence pattern in association with the identification information to the input terminal, and receiving, from the input terminal, a key code consisting of the numerals selected based on the shared rules from the random number sequence pattern by the user to be authenticated; and an authentication unit for authenticating the user to be authenticated, using the key code received and the key code generated.