The basis of this theory is that Tatsuhime, who was the third daughter of Mitsunari, was adopted by Kodaiin according to "Sugiyama-ke Yuishogaki (Pedigree of the Sugiyama Family)" and "Oka-ke Yuishogaki (Pedigree of the Oka Family)"; Kozosu, who was a top aid of Kodaiin, was Mitsunari's relative and negotiated to open Otsu Castle for the western army in the Battle of Sekigahara; and there were no Mitsunari's relatives in proximity to Yodo-dono (for details, see "Kodaiin").
その論拠として、三成の三女辰姫は高台院の養女である【杉山家由緒書・岡家由緒書】こと、側近筆頭の孝蔵主は三成の縁戚で関ヶ原でも西軍のために大津城の開城交渉を行っていること、淀殿の周辺に三成ら西軍派の縁者がいないことなどがあげられる（詳しくは高台院を参照）。