A professional competitor since 2000, he has formerly competed in PRIDE, Pancrase, UFC, Vale Tudo Japan, and RINGS.
[2] Kotani made his professional MMA debut in September 2000 for the RINGS promotion.
From 2004 to 2005, Kotani suffered a rough patch in his career as he went 3-5-1 against several UFC fighters including Roger Huerta, Yves Edwards, Marcus Aurélio and Rich Clementi.
In his debut, he faced Thiago Tavares at UFC Fight Night: Stevenson vs Guillard on April 5, 2007.
However, beginning in September 2010, Kotani started an undefeated streak of 13–0 over the next three and a half years in his native Japan.