A wireless IC device includes a wireless IC chip having a power supply circuit including a resonant circuit having a predetermined resonant frequency and a radiation plate that externally radiates a transmission signal supplied from the power supply circuit and that supplies a reception signal externally transmitted to the power supply circuit. The radiation plate is connected to the power supply circuit via an electric field or the radiation plate is coupled to the power supply circuit via a magnetic field. The radiation plate is a two-surface-open type radiation plate including at least one radiation portion arranged to externally exchange a transmission-reception signal and a power supply portion arranged to exchange a transmission-reception signal with the power supply circuit.