PURPOSE: In an insulating transformer comprising balanced and unblanced windings of an arbitrary turn ratio, to relieve the influence of longitudinal current by connecting a line type longitudinal retardation coil of a turn ratio 1/1 in series with the balanced winding and ground the middle tap of the balanced winding through a capacitor.

CONSTITUTION: An insulating transformer 3 comprises a balanced winding 6 and an unbalanced winding 8 of an arbitrary turn ratio. A line type longitudianl retardation coil 4 comprising two windings 10 and 11 of a turn ratio 1/1 is connected to the winding 6, and the middle tap 12 is grounded through a capacitor 13 of which the impedance is negligible. The longitudinal current IU flowing into the coil 4 from one side 14,15 is flowed to the earth through windings 16 and 17 and the capacitor 13, to cancel the magnetic flux developed in the windings 16 and 17. The potentials at the ends 19 and 20 of the winding 6 are nearly equal to the earth potential, so that the current IU does not affect the unbalanced winding 8. Thus there is provided a communication circuit of good longitudinal retardation characteristic.