A frequency dependency of PD signal propagation in GIS was investigated at an underground substation. Injecting dummy pulses into a GIS through its internal electrode, frequency spectrums were measured by a spectrum analyzer at various points of the GIS. It is shown that this measurement is very reproducible and the signal propagation phenomena are independent of its direction. It is also clarified that the PD signal reduction phenomena are deeply related with mode transfer from TEM to TE or TM. The major higher mode seems TE11. Pulse waveforms were also measured with a 65MHz bandpass filter. Their magnitudes are compared with the gain of 65MHz component of the frequency spectrums and good agreement is assured. This implies a possibility of PD location by observing a frequency pattern with several bandpass filters.