To solve the problem of a conventional balun transformer, wherein it is difficult to adjust the attenuation other than the pass band, depending on electronic apparatuses in use and cannot have obtained characteristics required for using the electronic apparatuses because the conventional balun transformer employs coils only electromagnetically coupled with each other.

The balun transformer disclosed herein includes first and second coils connected in series; a third coil coupled with the first coil and connected between a first balance terminal and ground; a fourth coil coupled with the second coil and connected between a second balance terminal and ground; a first capacitor connected between an end of the first coil opposite to a connecting point with the first coil and the ground side end of the third coil and connected to ground; a second capacitor connected between an end of the first coil opposite to a connection point with the first coil and the ground side end of the fourth coil and connected to ground; and a third capacitor connected between the first and second balance terminals. Either of the first and second coils is connected to an unbalanced terminal.