Officers took gangster Shingo Tamiguchi, 41, and six other male and female suspects into custody for violating of the Professional Motor Boat Racing Law regarding bookmaking.
At present, gambling on horse, boat, bicycle, and motorcycle racing from authorized betting outlets is legal in Japan.
Four customers were also arrested.
On that same day at around noon, officers arrived at a residence in Nishinari Ward and discovered a television connected to a broadcast of a boat racing event and the buying and selling of betting tickets.