The Soai-kai was formed in 1945 by Toramatsu Takahashi, then a member of a Yokohama-based bakuto group named the Sasada-ikka. Originally named the "Gyosui-kai" (魚水会?) as an affiliate of the Sasada-ikka, the group was renamed the "Takatora-gumi" (高寅組?) and again renamed the Soai-kai in 1955 when the group became independent from the Sasada-ikka.
Headquartered in Ichihara, Chiba, the Soai-kai is one of the three dominating yakuza groups in Chiba Prefecture, along with the Sumiyoshi-kai and the Inagawa-kai. The Soai-kai is a member of a bakuto fraternal federation named the Kanto Hatsuka-kai, along with four other Kanto-based yakuza syndicates, the Sumiyoshi-kai, the Inagawa-kai, the Matsuba-kai, and the Toa-kai.
- 1st: Toramatsu Takahashi
- 2nd: Takeo Azeta
- 3rd: Jiro Takahashi
- 4th: Yoshio Ishii
- 5th: Akira Takamura
- "Police of Japan 2011, Criminal Investigation : 2. Fight Against Organized Crime", December 2009, National Police Agency
- "2010 Police White Paper Chapter 2 : Furtherance of Organized Crime Countermeasures", 2010, National Police Agency (Japanese)
- "1993 Police White Paper Chapter 1 : The Actual Condition of the Boryokudan", 1993, National Police Agency (Japanese)
- "Condition of Boryokudan", as of late 2010, Chiba Prefectural Police (Japanese)
- "The Yamaguchi-gumi Bakuhu theory", Kenji Ino, 17 December 2007 (Japanese)