A filter using a transmission line formed by a cascade connection of invertedL-type networks has been developed to reject the distortion of the probe characteristics by rf (radio-frequency) noise. Each inverseLnetwork consists of two coaxial cables with the same physical constant and length. The filter can remove discrete frequency components including the fundamental and harmonic components, the cut-off frequencies being determined by the distributed circuit constant and the length of the cables. By inserting different kinds of the network in cascade, many noise components associated with the rf frequency can be eliminated at the end section of the filter. Experiments have been performed in rf plasmas by inserting three kinds of invertedLnetworks with the frequencyf(13.56 MHz),2 fand4fas the cut-off frequency. Distortion free probe characteristics have been obtained, from which accurate determination of plasma parameter such as the electron energy distribution is possible. ©1997 American Institute of Physics.