TOKYO (AFP) - A newly appointed female judge on Japan's Supreme Court has said she will use her maiden name when handing down rulings, a legal first in a country criticised for its attitudes to gender equality.

Married couples in Japan are required to have a common surname under a law that was upheld in 2015, sparking criticism from activists who complain it is sexist and outdated.

Social conservatives defend the law as crucial to maintaining Japan's traditional family structure but critics say it reflects a society that is still male-dominated and lags behind other advanced nations in terms of equality.

Yuko Miyazaki, 66, the country's sixth-ever female member of Japan's top court, confirmed to AFP through a spokesman that she "will use her maiden name" for judgements.

"It is natural for me to keep using the name I used as an attorney," she told local media, adding it was important to have the "option" of keeping a pre-marriage name as traditional values change.

The Supreme Court allows its officials to use pre-marriage names but Miyazaki is the first top court judge to choose to do so.

Japan ranked bottom of the G7 countries in the World Economic Forum's latest "Global Gender Gap Report", coming 114th worldwide.

It scored poorly on women's participation in the economy and political involvement, as only around 10 percent of the lower house of parliament is made up of female MPs.

Miyazaki said she was inspired to go into the law - at a time when it was difficult for women to find a job - by her father, who told her there was "no difference between men and women in court".

After graduating from the University of Tokyo's faculty of law in 1976 and Harvard Law School in 1984, she registered as an attorney before marriage with her maiden name Miyazaki.

She has won global recognition in legal circles as a corporate and tax lawyer.

But she was reportedly once turned away from a hotel in a foreign country because her professional name was different from her legal name.

'Individual dignity'

The surname law, which dates to 1898, is a throwback to the country's feudal family system, in which all women and children came under the control of the head of the household - overwhelmingly men.

That system was abolished in 1948 in broad reforms following the post-World War II US occupation, but Japan's civil code maintained the surname rule.

A December 2015 Supreme Court ruling said the law "does not violate the constitution", noting that changing one's name after marriage did not harm "individual dignity and equality between men and women", as maiden names can still be used informally.

Even though men can in theory take their wife's surname, in reality about 96 per cent of married women in Japan take their husband's family name.

Men in Japan occasionally take their wife's surname to maintain that name if her family has no male heir.

On Tuesday (Jan 9), plaintiffs including Yoshihisa Aono, the male president of major software firm Cybozu, filed a lawsuit seeking compensation for mental anguish for being unable to use their maiden names, hoping to overturn the 2015 Supreme Court verdict.

Aono legally registered his wife's surname - Nishihata - upon marriage, but he uses his unmarried name for business purposes.