In handing down the sentence, a judge at the court in Fukuoka, western Japan, said strict controls were in place at the Kudokai crime syndicate, and it was unthinkable that the attacks had been carried out without the leader's authorization, NHK said.

Between 1998 and 2014, Kudokai members shot and killed a former head of a fishing cooperative, and injured three others including a nurse and a former police officer by shooting or stabbing, NHK said.

Defense lawyers for the Kudokai head, Satoru Nomura, 74, plan to appeal the ruling, Kyodo news agency said.