The characteristic impedance of traveling wave tube is characterized by the ratio of the square of the electric field to the power. But in this paper, the input impedance of helix defined as the ratio of the voltage to the current is considered and measured. This impedance plays a major role in the matching of helical type delay lines. The input impedance of helix has been analyzed in our previous paper. But the measurements for it, are not so easy that the experimental researches are very rare, because of their poor reproducibility and large errors. The mean diameter of the helix used in the measurement is 4.0mm, and values of the ratio of the conductor diameter to the pitch are between 2 to 5. Frequencies for the measurement are in the 4000 Mc band, and the measuring apparatus are all of the coaxial type. In order to prevent the axial component of the electric field of the helix from being disturbed, the diameter of the outer cylindrical conductor is chosen to be more than three times as large as that of the helix. The input impedance of the helix is much larger than the impedance of a coaxial type standing wave detector. Accordingly it is possible to make the standing wave ratio smaller by using an impedance transformer. The results of experiments show that the real part of the input impedance decreases with increased frequency, and its imaginary part has the same characteristics as those of the real part but are negative in value. The polarity of the imaginary part obtained by experiments coincides with that theoretically obtained through equivalent analysis for the distribution line, while in its frequency characteristics no coincidence is observed between the two. The tendency of the experimental values coincides with that of the theoretical ones. Experiments show that the measurement has a good reproducibility of its results, the maximum error for the real part being within 15 percent. This data is useful in designing the junction between a waveguide and a traveling wave tube, which will be discussed in the next report.