On an adapter mounting portion 3a having a projecting cross section which is formed on a cap 3 of a small-sized memory card 1, a recessed portion of an adapter 2 side is fitted so that both parts are formed as an integral unit in a replaceable manner. Accordingly, the small-sized memory card 1 can maintain the dimensional compatibility with respect to existing memory cards whereby the small-sized memory card 1 can be used also in equipment which is designed to cope with the existing memory cards.