Legal experts said that even if the Supreme Court upheld the earlier ruling, it would not upend Prime Minister Shinzo Abe’s landslide victory in those elections, because it would affect a limited number of electoral districts. But it could be seen, they say, as a signal that Japan’s long-passive courts may be ready to force lawmakers to fix voting imbalances.

“There is definitely a shift in judicial thinking toward one person, one vote, and Mr. Masunaga is a major force behind that,” said Yasuo Hasebe, a constitutional scholar at the University of Tokyo.

Mr. Abe is already feeling that pressure; his party has put forth a bill to eliminate five rural Parliament seats to reduce voting disparities, though experts say it does not go nearly as far as one person, one vote.

Mr. Masunaga, who studied law at Columbia University in the late 1970s and became a licensed lawyer in New York, said he was inspired to act in Japan by what he saw in the United States. He was especially impressed by a series of Supreme Court rulings in the early 1960s that made one person, one vote a cornerstone of the civil rights movement, ending the use of unfairly drawn election districts to disenfranchise black voters.

“Americans know that democracy is something that must constantly be fought for,” said Mr. Masunaga, who lived in the United States for six years. “Japan never had a revolutionary war. Democracy was given to us by MacArthur,” he added, referring to Japan’s Constitution, which was written by postwar American occupiers under Gen. Douglas MacArthur.

Mr. Masunaga said his time in the United States also gave him an appreciation for individual rights, which are often weak in Japan. Until the March court victory, Mr. Masunaga was best known for a landmark case eight years ago that was also seen as groundbreaking for seeming to strike a blow at Japan’s vaunted group loyalty. The courts awarded his client, a corporate engineer, $8 million in royalties for his invention of the blue diode light, the basis of white LED light bulbs. It was the first time a Japanese salaryman had won such a legal claim against his employer.