PURPOSE: To prevent noise from being invaded, by using a balun transformer having a metallic powder type core for a receiving end of a digital line tying a plurality of information processors.

CONSTITUTION: At a receiving end of a line tying a plurality of information processors, e.g., at the end of lines 3a, 3b tying information processors 1a, 1b, balun transformers 5, 6 are inserted. In the balun transformers 5, 6, coils 5a, 5b and 6a, 6b are wound on one iron core of metallic powder type having worse high- frequency characteristics, using iron-silicon-aluminum alloy as raw materials and the lines 3a, 3b are connected. In the balun transformer, no impedance is shown to the input in opposite phase and the loss due to eddy current is increased to the input in phase according to the frequency increase. Thus, the noise in high frequencies invaded to lines can be rejected easily.