An authentication section comparing a characteristic included in a user's
input into an input section with a characteristic of a person registered
in memory, to determine an authentication result. An authentication
control section including a function for (i) storing a value indicating a
likelihood of being the person, (ii) controlling the value indicating a
likelihood of the user being the person, based on (i) the authentication
result of the authentication section and (ii) a passage of time. In
response to an instruction to begin a process which requires
authentication, the authentication control section controls whether or
not to permit execution of the process depending on a the stored value
Thus, unauthorized use of a device is prevented, while the trouble of
asking a user to make a special input for authentication is resolved when
a process requiring authentication is executed.