The Diet has passed a bill to slash the number of lower house seats by 10 to 465 and rezone electoral districts in an effort to address the vote weight disparities among different regions.
The new law, enacted by a majority vote in the House of Councillors on Friday, calls for fewer House of Representatives members by cutting six single-seat constituencies and four proportional representation seats.
As redefining the electoral districts will require some time, an election under the amended system can only be conducted next year at the earliest. If an election is called before the procedure is completed, it will be held under the pre-reform system.
The latest change is intended to keep the vote weight disparities between the most and least populated single-seat constituencies below twofold—a threshold that when exceeded is seen by the Supreme Court as problematic from a constitutional point of view.
The single-seat constituencies to be cut are in Aomori, Iwate, Mie, Nara, Kumamoto and Kagoshima prefectures, while the four proportional representation seats are from blocs in Tohoku, Hokuriku-Shinetsu, Kinki and Kyushu.
To better reflect the population of prefectures and regional blocs, the new law also calls for introducing a seat allocation method later based on a formula believed to have been proposed by John Quincy Adams, the sixth president of the United States.
But as the introduction will be based on the results of the 2020 national population census and subsequently unrealized until 2022 or later, opponents who sought immediate introduction of the so-called Adams method remain critical, saying a court could still rule against elections held under the new law.