The authors have been examining the diagnosis method for partial discharge (PD) occurring inside gas insulated switchgear (GIS) using the UHF method with a focus on electromagnetic (EM) wave mode conversions at L-shaped, T-shaped pipes, and components existing in actual GIS. Based on these conversion characteristics, an on-site, high-accuracy PD diagnosis algorithm was constructed for use on high-order mode EM-waves. All the researches so far were targeted at an isolated-phase-type (singlephase- type) tank. The present study was targeted at a straight-pipe three-phase structure, the most basic form, in order to apply the evaluation method to a three-phase-common-type tank (three-phase-type tank). Through the implementation of EM-wave propagation experiments and the EM-field analysis, the presence of TEM, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> modes was verified also in the three-phase-type tank. Next, the EM-field analysis was conducted on models equivalent to actual apparatuses of 72 to 550 kV classes. Then, formulas for simplified calculation of cutoff frequencies in TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> in a three-phase-type tank were proposed. Furthermore, the basic concept of the PD diagnosis method was examined and the work flow was presented.