Officers took Mitsuo Seki, a 60-year-old boss in the Sumiyoshi-kai, into custody for allegedly recommending customers to a baccarat casino previously busted by police on charges of operating a wagering facility for profit.
In the earlier bust, police arrested five employees, including the manager, 46-year-old Keisaku Kimura.
All of the suspects have reportedly denied the allegations.
The casino opened in November. Between February and April, the operation collected 100 million yen in revenue.
Seki is believed to have provided an introduction to approximately 10 percent of the 380 people included on a customer list seized by police.
According to police, a portion of the funds obtained through the casino was being used by the organized crime group.