To provide a wireless IC device capable of controlling the gain of transmission and reception signals. A wireless IC device includes a wireless IC chip (5) processing a certain radio signal; a power-supply circuit board (4) that is connected to the wireless IC chip (5) and that includes a power supply circuit including at least one coil pattern (23); and a radiation plate (3) radiating a transmission signal supplied from the power-supply circuit board (4) and/or receiving a reception signal to supply the reception signal to the power-supply circuit board (4). The radiation plate (3) includes an opening (7) provided in a part thereof and a slit (6) linking to the opening (7). Viewed in plan from the direction of the winding axis of the coil pattern (23), the opening (7) in the radiation plate (3) is overlapped with an inner area of the coil pattern (23) and the area of the inner area is approximately the same as that of opening (7).