Apparatus and a method for preventing unauthorized use and a device with a function of preventing unauthorized use

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.