To provide a balun transformer adaptable to signals in two or more frequency bands.

The transformer comprises an unbalanced transmission line 10, a balanced transmission line 20, an unbalanced side impedance adjusting means 40 and a balanced side impedance adjusting means 50, thus adapting to signals in two or more frequency bands. The unbalanced transmission line 10 is coupled with the balanced transmission line 20. The unbalanced side impedance adjusting means 40 has an unbalanced side control terminal T41 and is connected to the unbalanced side transmission line 10 to change an impedance characteristic, based on a control signal fed to the control terminal T41.The balanced side impedance adjusting means 50has a balanced side control terminal T51 and is connected to the balanced side transmission line 20 to change the impedance characteristic, based on a control signal fed to the control terminal T51.