The Tokyo High Court ruled Thursday that the House of Representatives election in December, which had a maximum 2.13-fold disparity in the weight of votes between districts, was constitutional.
With the ruling, the court dismissed plaintiffs' demand that the election results in 11 central and eastern Japan prefectures including Tokyo be nullified.
The decision is the first ruling among a series of lawsuits on last December's snap general election.
A group of lawyers led by Hidetoshi Masunaga had sought to have the election results nullified, claiming that the Constitution's guarantee of equality under law was violated by significant variation in the population of voters in single-seat districts.
In Thursday's ruling, Presiding Judge Toru Odan said the vote-weight disparity in the general election cannot be regarded as unconstitutional, pointing out that the gap has been narrowed to somewhat over twofold due to such factors as a cut in the number of single-seat constituencies to 295 from 300.
"Efforts by the Diet to redress the situation were appropriate," Odan said.
According to government data, the vote weight gap in the December 2014 election was 2.13-fold between Tokyo's No. 1 district, the country's most populous single-seat constituency, and Miyagi Prefecture's No. 5 district, the least populous.
The Supreme Court ruled in November last year that disparities in the weight of votes in the previous Lower House election in December 2012 that were as great as 2.43-fold were "in a state of unconstitutionality."
A decision on a lawsuit filed by a separate group at the Tokyo High Court seeking the nullification of the 2014 election results in eight districts in Tokyo and neighboring Kanagawa Prefecture is set to be announced on March 25.
Lawsuits involving the validity of national elections results bypass district courts and are initially tried at a high court, under the public offices election law.