To prevent an input password from being stolen by an invalid authentication device. An authentication device 10 for authenticating an inputter based on an input password accepted from the inputter is provided with a first input part 110 for allowing a first part of the input password to be inputted; a confirmation information output part 120 for outputting confirmation information known to the valid inputter in advance when the first part 10 is valid, the confirmation information indicating to the user that the authentication device is valid; a second input part 130 for allowing a second part following the first part of the input password to be inputted; and a determination part 140 for determining that the inputter is valid when the first part and the second part are valid.