PURPOSE: To reduce the man-hour of production and to realize the function of a balun transformer with a low cost by forming patterns of both faces of a printed board and combining transformers by a board side coupling line to constitute the balun transformer.

CONSTITUTION: The signal inputted from a terminal 1 by a micro stripliney being an unbalanced line flows onto patterns 6 and 7 forming an isolation transformer 4 as differential currents which nave the same magnitude but flow in opposite directions. If coupling of the transformer 4 is weak, an in-phase current is generated in the pattern 7 in the second face layer, and differential currents don't flow. A potential setting phase inversion transformer 5 consisting of patterns 8 and 9 functions to stop this in-phase current. Balanced output signals which have the same magnitude and have opposite phases are obtained in output terminals 2 and 3 by this function of the potential setting phase inversion transformer 5, and this device functions as the balun transformer.