PURPOSE: To obtain the constant-impedance filter whose various characteristics are excellent by forming the noise filter of an inductance, serial resistances, a capacitor, etc., and selecting the resistances, inductance, capacitor, etc., so that they have a necessary relation.

CONSTITUTION: Between an input terminal 1 and an output terminal 3, the inductance L and two resistances R parallel to it are connected and among the connection point of the resistances R, an input terminal 2 and an output terminal 4, the capacitor C is connected to form the noise filter. When the inductance L, resistances R, and capacitor C are so selected as to satisfy R=(L/C)2/1, the filter has constant impedance between the input and output, the impedance viewed from the line becomes R, and the constant impedance performs the same operation with the resistances; when an noise current flows in, loss is caused to reduce its energy, and the filter does not resonates. Therefore, the noise filter which has various excellent characteristics and can prevent noise generation becomes the constant-impedance filter.