To solve the problem in that an insertion loss is low at a frequency higher than a given frequency and noises with frequency higher than a working frequency band is also produced.

The balun transformer includes first and second lines connected in series between an unbalanced terminal and a dummy terminal, a third line joined electromagnetically to the first line and connected between a first balanced terminal and the ground, and a fourth line electromagnetically joined to the second line and connected to a second balanced terminal. A first parallel circuit made up of a first inductor and a first capacitor connected in parallel is connected between an input/output terminal and the unbalanced terminal. A second parallel circuit made up of a second inductor and a second capacitor connected in parallel is connected between one end of the first parallel circuit and the ground. As a result, noise removing performance other than the working frequency band can be improved.