To provide a planar balloon transformer having a desired impedance ratio without complicating a working technique.

The balloon transformer includes first and second ground potential layers 20, 40 formed on a silicon substrate 10, an unbalanced signal first transmission line 50 having a dielectric layer 30 interposed between the layers 20 and 40 and provided in the layer 30, and a second transmission line 60 and a third transmission line 70 vertically arranged in parallel with the layers 20 and 40. The thickness of the layer 30, the intervals and the offset of the line 50, the line 60 and the line 70, the thicknesses of the lines, and the relative dielectric constants of the dielectrics have a predetermined relationship.