PURPOSE: To suppress harmful parasitic oscillation and to improve the efficiency of oscillating circuit by a method wherein two coils are wound so that their electromotive force may be induced to be in-phase and leakage inductance is reduced.

CONSTITUTION: In a primary side of a transformer two coils 1, 2 which have different number of turns and have a common terminal 3 at the beginning of windings (or at the end of windings) are wound so that they may have in-phase induced electromotive force. The other terminal of a coil 1 which has less number of turns is assign to an AC grounding point 5 and among another terminal 4 of coil 2 which has larger number of turns and the common terminal 3 one of them is chosen to be a driving terminal and another terminal is assign to a feed back terminal.