The Supreme Court ruled Wednesday that disparities of up to 2.43 times in the weight of votes in last December's House of Representatives election were "in a state of unconstitutionality."
But it stopped short of declaring the election outcome violates the supreme law and should be invalidated.
The Supreme Court's grand bench handed down its decision after high courts across Japan ruled the vote disparities were unconstitutional, with two rulings even nullifying the election as demanded by two lawyers' groups led by Kuniaki Yamaguchi and Hidetoshi Masunaga.
The top court's latest ruling followed its decision in March 2011 that the gap of up to 2.3 times in the weight of votes in a 2009 lower house election was "in a state of unconstitutionality."
Wednesday's ruling said the vote-value gap in the 2012 election was "extremely unequal" as in the 2009 race, but that there was not enough time to correct the gap.
The Supreme Court noted that parliament "achieved certain progress" in correcting the vote-value gap through a law revision but that the structural problem has not been fully resolved, and urged the Diet to continue electoral reform efforts.
Three out of 14 Supreme Court justices expressed their opposition to the latest decision, claiming the zoning of electoral districts in the 2012 election violated the Constitution.
Prime Minister Shinzo Abe told reporters that he takes the ruling seriously and will closely examine it.
Parliament enacted a bill last November to reduce the disparity by cutting the number of single-seat constituencies in Japan to 295 from 300. But the general election was held the following month with the same zoning of electoral districts as in the 2009 contest, as there had not been enough time to make the changes.
As a result, the gap in the vote-value widened in the last general election to up to 2.43 times. The ballot count was lowest in the Kochi No. 3 district and highest in the Chiba No. 4 constituency.
High courts have ruled in 12 cases that the December election results were unconstitutional but valid, while two rulings said outcomes were in a state of unconstitutionality. Two other rulings invalidated results as unconstitutional for the first time in postwar Japan.
In June, the revision of the Public Office Election Law took effect, altering the zoning of electoral districts and narrowing the vote disparity to 1.998 times.
In 1976 and 1985, the Supreme Court ruled that disparities of up to 4.99 times and 4.40 times in lower house elections conducted under multiple-seat electoral system in 1972 and 1983, respectively, were unconstitutional, but fell short of declaring the results invalid.