An antenna-electrode structure includes a feeding radiant-electrode and a grounded portion arranged such that an open-end of the feeding radiant-electrode defines a capacitance to the grounded portion therebetween, and a non-feeding radiant-electrode arranged to electromagnetically couple the feeding radiant-electrode. The non-feeding radiant electrode is arranged such that an open-end thereof defines a capacitance to the grounded portion therebetween to produce a dual-frequency resonance state together with the feeding radiant-electrode. In response to a signal supplied from a signal-supply source, the feeding radiant-electrode performs an antenna action and the non-feeding radiant-electrode in turn performs an antenna action by signal transmission from the feeding radiant-electrode, such that by being excited from these actions, the grounded portion also performs an antenna action. Because of the antenna action of the grounded portion, the sizes of the feeding radiant-electrode and the non-feeding radiant electrode are reduced.