1. An autofocus system comprising: an imaging unit that takes a subject image formed by an optical system; an autofocus unit that performs focus adjustment of the optical system so as to bring into focus a subject in a predetermined AF area in a shot image taken by the imaging unit; a pattern registration unit that registers pattern data for specifying a subject; a tracking target detection unit that detects a tracking target subject among subjects in the image taken by the imaging unit, based on the pattern data registered in the pattern registration unit; a tracking unit that moves the AF area in accordance with movement of the tracking target subject detected by the tracking target detection unit, wherein the tracking target detection unit includes a subject detection unit that detects at least one subject that is of a same type as the subject of the pattern data registered by the pattern registration unit, among the subjects in the image taken by the imaging unit, and the tracking target detection unit detects the tracking target subject among the at least one subject, which is detected by the subject detection unit, based on the pattern data registered by the pattern registration unit; a title registration unit that registers a title for identifying the subject of which the pattern data is registered when the pattern data is registered by the pattern registration unit; and a title display unit that, when the tracking target subject is detected by the tracking target detection unit, displays the title registered by the title registration unit in a screen of a viewfinder for displaying a currently shot image so that the displayed title corresponds to the detected subject, wherein the pattern registration unit registers plural pieces of pattern data for specifying a plurality of subjects, respectively, the title registration unit registers a plurality of titles for identifying the plurality of subjects of which pattern data are registered, respectively, the tracking target detection unit further includes a registration subject detection unit that detects at least one subject, which is registered by the pattern registration unit, from among the at least one subject, which is detected by the subject detection unit, based on the pattern data, which is registered by the pattern registration unit, and a tracking target determination unit that determines the tracking target subject from among the at least one subject detected by the registration subject detection unit, the tracking unit moves the AF area in accordance with movement of the tracking target subject determined by the tracking target determination unit of the tracking target detection unit, and the autofocus system further comprises: a frame display unit that displays a frame surrounding the at least one subject, which is detected by the subject detection unit, on the screen of the viewfinder, wherein the title display unit displays on the screen of the viewfinder the title, which is registered by the title registration unit, with respect to the at least one subject, which is detected by the registration subject detection unit, and the title display unit displays on the screen of the viewfinder information that indicates non-registration with respect to the subject, which is detected by the subject detection unit and is not registered by the pattern registration unit, and an instruction unit that turns on/off displaying the title and displaying the information, which indicates non-registration, by of the title display unit.