Six couples have filed lawsuits against Japan's government, claiming the country's laws that ban the use of different surnames for married couples violate the Constitution.

The one married and five unmarried couples living in Tokyo, Hokkaido and elsewhere are demanding damages from the government in the lawsuits filed at courts in Tokyo and Sapporo on Friday.

The plaintiffs and their defense counsel say changing surnames for marriage makes it difficult to maintain credibility and appraisal linked to birth names. They also say changing surnames can cause identity crises for some people.

They said couples who stay unmarried to keep their separate surnames often face disadvantages including those involving inheritance after a partner's death.

Similar lawsuits have been filed in the past. In 2015 and 2021, the Supreme Court's 15-member grand bench ruled the laws constitutional by majority vote. But five justices in 2015 and four in 2021 called the laws unconstitutional.

One of the plaintiffs said he has a daughter in junior high school, but cannot receive legal benefits because he is unmarried. He said he does not want his daughter's generation to live with an unreasonable system in which a husband or wife must give up his or her surname to get married.