A balun device including a first to a third inductance elements and a first to a third capacitance elements is provided. The first inductance element has a first end for receiving an input signal and a second end. The second inductance element has a third end and a fourth end, wherein the third and the forth ends are for outputting a first and a second output signals corresponding to the input signal, respectively. The first output signal and the second output signal substantially have the same amplitude and opposite phases. The first and the second inductance elements generate mutual inductance. The first capacitance element is coupled to the first end. The second capacitance element is coupled to the third end. The third capacitance element is coupled to the fourth end. The third inductance element is seriesly connected to one of the first to the third capacitance elements.