To provide a balun transformer of of wide band which is low in profile.

In the balun transformer, a first transmission line, where unbalance signal is transferred and second and third transmission lines where balance signal is transferred are arranged inside a laminate in which three or more dielectric layers, is laminated so that at least a part of them face each other across the dielectric layers. A first ground electrode which is electromagnetically coupled with the first transmission line is coated on one main surface of the laminate, while a second ground electrode which is electromagnetically coupled with the second and third transmission lines is coated on the other main surface. The relative dielectric constant of at least one of the dielectric layer between the first transmission line and first ground electrode, and that between the second and third transmission line and second ground electrode is set to be lower than the relative dielectric constant of the dielectric layer between the first transmission line and the second and third transmission lines.