A group of lawyers filed lawsuits across Japan on Monday seeking nullification of the outcome of Sunday's House of Councillors election due to the uncorrected disparity in the weight of votes among constituencies.
The move at the country's 14 high courts and branches marks the first time that lawsuits have been filed to challenge the results of all districts in an election.
Another group of lawyers separately brought a case to the Hiroshima High Court in the morning seeking to invalidate the result for the Hiroshima prefectural district, and is planning to take a similar action in Tokyo later.
The lawyers argue that the disparity in the value of votes in Japan's 47 prefectural electoral districts is unconstitutional.
"The latest election was held without drastic reforms," Tetsuya Kanao, who is part of the group that filed the suit in Hiroshima, said at a news conference.
"It is natural for the number of lawmakers to be in proportion with the population in a democratic, advanced country and it is only in Japan that such a disparity exists," Kanao said.
Based on the number of voters on Sunday, the value of a vote in Tottori, the least populated per lawmaker, was 4.77-fold greater than in Hokkaido, the most.
The Supreme Court ruled last October that the maximum fivefold disparity in the weight of votes in the 2010 upper house election was "close to unconstitutionality." The top court urged lawmakers to promptly correct the disparity.
Amendments made last November to the election law, effective from the just-ended election, only sliced the disparity by adding two seats each to two populated districts and reducing two each in two others.
Lawsuits have similarly been filed over vote disparities in the House of Representatives election in December, when the Liberal Democratic Party overthrew the Democratic Party of Japan and returned to power.
The December election was conducted with the same zoning of electoral districts as in the 2009 contest although the law enacted in November to reduce disparities requires that the number of lower house single-seat constituencies be cut to 295 from 300.
The Supreme Court is expected to make its judgment on the matter within the year. High courts have ruled in 12 cases that the election results were unconstitutional but valid, while two rulings said results were close to unconstitutionality. Two ruling invalidated results as unconstitutional for the first time.