An antenna structure includes four induction antennas which have the same structure, are connected in parallel and are disposed to be overlapped. The induction antennas include an external upper section arranged on a first quadrant of a first layer, an internal upper section connected to the external upper section and arranged on a second quadrant of the first layer, an internal lower section connected to the internal upper section and arranged on a third quadrant of a second layer arranged on a lower part of the first layer, and an external lower section connected to the internal lower section and arranged on a fourth quadrant of the second layer. An RF power is supplied to one end of the external upper section, and the other end of the external lower section is grounded.