PURPOSE: To improve the amplitude-frequency characteristic and return loss characteristic up to a high frequency region by twisting the winding lead with the one end grounded while the other end opened and other desired number of winding leads together.

CONSTITUTION: The primary and secondary signal winding leads 8, 9 has the winding ratio of 1:1 and each signal winding lead is turned four times around the troidal core using the ferrite material. The earth winding lead 5 is twisted with the signal winding lead and turned four times. According to this structure, the leakage inductance of signal winding lead is cancelled by the static capacitance like a distributed constant generated between the signal winding lead and earth winding lead and thereby the amplitude-frequency characteristic and return loss characteristic in the frequency band up to a radio frequency can be improved.