Japanese prosecutors have demanded a four-year prison term for former justice minister Kawai Katsuyuki for what they called an "unprecedented" act of vote-buying by a Diet member.

Kawai is accused of bribing local politicians in Hiroshima Prefecture to help his wife, Kawai Anri, win a seat in the 2019 Upper House election.

In a court session on Friday, prosecutors demanded that Kawai Katsuyuki serve four years in prison and forfeit 1.5 million yen, or about 13,700 dollars.

Prosecutors said Kawai effectively managed his wife's election campaign, as he was aware of her campaign staff's activities and gave them instructions.

They also said Kawai and his wife acted in conspiracy to commit a crime by closely sharing information.

Prosecutors said the act of vote-buying was masterminded by a Diet member. They said it was "unprecedented, exceptionally malicious, and tarnished public trust in the country's elections."

Defense lawyers are due to deliver their closing arguments on May 18.

Kawai has given up his Lower House seat. His wife was found guilty in January and has resigned from the Upper House.