A method of setting function includes first to fourth steps. The first step connects a recording medium to a first electrical apparatus having an optional function either disabled or temporarily enabled, the recording medium containing license information to enable the optional function. The second step allows first electrical apparatus to authenticate the license information. The third step, subsequent to the second step, enables the optional function of the first electrical apparatus. The fourth step, subsequent to the third step, records the optional function as having been authenticated in the license information.