An authentication device 1 includes: a display input device 3 that obtains numerical values of plural number keys, which numerical values are entered simultaneously via the number keys; a calculating section 9 that determines an input password by calculating, in accordance with a predetermined calculation method, the plural numerical values entered into the display input device 3; and a password verifying section 10 that verifies the input password determined by the calculating section 9 against a predetermined verification password. This make it possible to prevent, while keeping the passwords easy for the users to memorize, third parties from stealing the passwords by looking at how the users move their fingers and arms, and also to prevent the third parties from misusing the passwords even if the third parties figure out the passwords through memos or the Internet.