A resonant tag has one electrode plate of a capacitor and an electric circuit, which is electrically connected to the capacitor, formed on one surface of the insulating film composing the resonant circuit. On the other surface of the insulating film, the other electrode plate of the capacitor, which is electrically connected to the electric circuit, is formed. Heat pressing is performed on the insulating film existing between the two electrode plates with a predetermined pressure and at a predetermined temperature to shorten a distance between these electrode plates, and a crystal structure of the insulating film is destroyed to form a penetrating hole which penetrates through both the electrode plates.