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It is an effective method of insulation accident prevention for gas-insulated switchgear (GIS) to detect a partial discharge (PD) that occurs as a preceding phenomenon for an insulation accident. Among various methods, a PD detection method based on the detection of electromagnetic emission through measurement of the UHF band (300 MHz to 3 GHz), which is relatively free of influences from external noises, is drawing attention as an effective method in terms of extensive and highly sensitive detection. This paper describes conducting EM-wave propagation analysis on GISs with L-sections using the finite difference-time domain (FD-TD) method and examining the influences of the Insertions on the propagation characteristics of EM-wave modes with comparison of straight GIS part. As a result, this study identified that, even on a straight GIS, the velocity dispersion characteristic of high-order modes resulted in a decrease in the crest value even if there is no reflection or conductor loss in the discontinuities. And that mode conversions and reflections in L-sections also resulted in a decrease in the crest value.