A balun includes a first, second, and third metallic layers, a first dielectric layer disposed between the second and third metallic layers, and a dielectric substrate. The second metallic layer includes a first spiral line having sequentially connected first line segments and a second spiral line having sequentially connected second line segments. A first distance between each two opposite sides of a first region encircled by the innermost first line segments is greater than a second distance between each two adjacent parallel first line segments. A third distance between each two opposite sides of a second region encircled by the innermost second line segments is greater than a fourth distance between each two adjacent parallel second line segments. The third metallic layer includes a third and a fourth spiral lines. The first metallic layer and other elements as a whole are disposed on an opposite surface of the dielectric substrate.