A 3-line balun transformer which has a simple structure and is easy to design and manufacture. The balun transformer comprises an unbalanced port for inputting or outputting an unbalanced signal, first and second balanced ports for outputting or inputting balanced signals, respectively, the balanced signals being the same in level and 180 degrees out of phase with each other, a first line having its first end connected to the unbalanced port and its second end connected to ground, a second line arranged in parallel with the first line while being spaced apart from the first line by a predetermined distance, the second line having its first end and its second end connected to the first balanced port, and a third line arranged in parallel with the second line while being spaced apart from the second line by a predetermined distance, the third line having its first end connected to the first end of the second line and its second end connected to the second balanced port.