A group of lawyers and voters has filed a lawsuit demanding that the results of Sunday's Upper House election be nullified, due to large disparities in the value of votes.
The group filed the suit with the Sapporo High Court in Japan's northern prefecture of Hokkaido on Monday morning.
Votes in the most populated electoral districts had about a third the value of those in the least populated districts in the recent poll.
The plaintiffs and another group say the large disparity violates the Constitution, which guarantees that votes have equal value.
The two groups plan to file similar suits with high courts and their branches across the country on Monday.
In the previous Upper House race three years ago, the vote gap was up to 3.08 times.
The Supreme Court ruled in 2017 that such a disparity is not unconstitutional. But it acknowledged the need to narrow the gap in the run-up to the next election.
Last year, the Diet revised the public offices election law to raise the number of seats up for grabs in Saitama from three to four. The number of voters per lawmaker had been the highest of any district.