A password authentication device comprising: a display device having a display screen; a display control unit configured to cause a plurality of input key groups (T1 to T3) including a plurality of soft keys for inputting a password for user authentication to be displayed on the display screen; an input operation unit provided at a different position from the plurality of input key groups (T1 to T3), and operated by a user to select a soft key on the display screen so as to enable input of a password; a sight line detection sensor configured to detect a direction of a line of sight of the user who operates the input operation unit; an operation target specifying part configured to specify an input key group (T1 to T3) located at an end of the line of sight of the user among the plurality of input key groups (T1 to T3), based on the direction of the line of sight detected by the sight line detection sensor; a password acquisition part configured to assume an operation in the input operation unit as an operation for the input key group (T1 to T3) specified by the operation target specifying part and to acquire a password inputted through the input operation unit; a password authentication part configured to authenticate the password acquired by the password acquisition part; and a person detection unit configured to detect whether a person is present around the user who is using the password authentication device, wherein, when no person is detected by the person detection unit, the display control unit displays only one input key group without performing a process of displaying the plurality of input key groups (T1 to T3) on the display screen, and the password acquisition part assumes an operation in the input operation unit as an operation for the one input key group displayed on the display screen and acquires a password inputted through the input operation unit. a display device having a display screen; a display control unit configured to cause a plurality of input key groups (T1 to T3) including a plurality of soft keys for inputting a password for user authentication to be displayed on the display screen; an input operation unit provided at a different position from the plurality of input key groups (T1 to T3), and operated by a user to select a soft key on the display screen so as to enable input of a password; a sight line detection sensor configured to detect a direction of a line of sight of the user who operates the input operation unit; an operation target specifying part configured to specify an input key group (T1 to T3) located at an end of the line of sight of the user among the plurality of input key groups (T1 to T3), based on the direction of the line of sight detected by the sight line detection sensor; a password acquisition part configured to assume an operation in the input operation unit as an operation for the input key group (T1 to T3) specified by the operation target specifying part and to acquire a password inputted through the input operation unit; a password authentication part configured to authenticate the password acquired by the password acquisition part; and a person detection unit configured to detect whether a person is present around the user who is using the password authentication device, wherein, when no person is detected by the person detection unit, the display control unit displays only one input key group without performing a process of displaying the plurality of input key groups (T1 to T3) on the display screen, and the password acquisition part assumes an operation in the input operation unit as an operation for the one input key group displayed on the display screen and acquires a password inputted through the input operation unit.