A thin film balun of the present invention comprises: an unbalanced transmission line UL including a first line portion L1 and a second line portion L2; a balanced transmission line BL including a third line portion L3 and a fourth line portion L4 that are positioned facing the first line portion L1 and the second line portion L2 and electromagnetically coupled to the first line portion L1 and the second line portion L2, respectively; an unbalanced terminal UT connected to an end of the first line portion L1; a first balanced terminal BT1 connected to the third line portion L3; a second balanced terminal BT2 connected to the fourth line portion L4; and a ground terminal G connected to the third line portion L3 and the fourth line portion L4, wherein the ground terminal G has an extension that extends from the ground terminal G to an area at the unbalanced terminal UT side.