The Social Democratic Party said Sunday it will leave the tripartite ruling coalition after its leader, Mizuho Fukushima, was kicked out of the Cabinet last week for opposing its decision to keep a contentious U.S. military base in Okinawa Prefecture.

After meeting with the SDP's regional chapter chiefs the same day, Fukushima, who was fired as consumer affairs minister Friday, said the decision was difficult to make but resolute.

"The SDP will remain true to its beliefs and will not take back the vows that we made," she said, referring to the small party's promise to remove U.S. Marine Corps Air Station Futenma from Okinawa.