A radio frequency IC tag includes an antenna substrate accommodated on a bottom portion of an outer casing and an antenna radiation portion mounted on a surface of the antenna substrate. These parts are encapsulated by a sealing agent. An IC chip is disposed at a substantial center on a backside of the antenna substrate. A ground pattern connected to an electrode terminal of the IC chip is formed in an area, on which the IC chip is not mounted. A through electrode passes through the antenna substrate from its back surface to above its front surface at a position slightly offset from the center thereof. The through electrode is connected to the antenna radiation portion on the front surface and to the electrode terminal of the IC chip on the back surface.