Ms. Uozumi, 36, said she did not want customers to confuse her with her husband, Shigeru Otsuka, as most employees are known primarily by their last names in Japan.
Using her birth surname went beyond convenience. It was also a declaration of empowerment.
“I feel more independent,” Ms. Uozumi said. “I feel more who I am.”
The choice she made, however, is not available to all Japanese women. This month, a Tokyo District Court declined to grant a high school teacher’s request to use her original name at work.
That decision came after Japan’s Supreme Court ruled in December that the law did not violate the Constitution or place an undue burden on women because an increasing number of employers now permit women to use their birth surnames professionally.
Critics were disappointed by the Supreme Court decision because it did not strike down the legal prohibition against separate surnames for married couples, leaving it to the Parliament instead.
Among democratic countries in the developed world, Japan ranks low on gender equality in health, education, the economy and politics. Despite recent high-profile examples, women hold very few powerful positions in politics or business, while many working mothers complain that day care is inadequate.
The marital naming law, supported by many conservatives who believe that women belong predominantly in the home supporting their husbands and families, is seen by some as another vestige of discrimination against women in Japanese society.
In the Tokyo District Court case, the three judges, all men, ruled that the teacher’s employer, a private school in Tokyo, could not be compelled to let her use her birth surname at work. Citing surveys that show about a quarter of women use their original surnames in the workplace, the court said doing so was “not deeply rooted in society.”
The plaintiff, who has remained anonymous in the publicly available court documents, declined an interview request through her lawyer. In court filings, the teacher, described as recently married, said students and colleagues knew her by her given surname. She asked to be allowed to continue to use it on letters home to parents, attendance records and report cards.
Makiko Terahara, one of the lawyers for the plaintiff, said the teacher, now in her 30s, had worked for 15 years using her birth surname.
“We have to work on changing the law so that couples can choose their names when marrying,” Ms. Terahara said. “A name represents a person. It’s a matter of personal rights.”
Hidehisa Takase, the principal of Nihon University Daisangakuen, the school where the plaintiff works, said he had offered a compromise in which the teacher could continue to be called by her birth name publicly but use her married name for official documents, including student report cards.
“We don’t think we denied her individual identity,” Mr. Takase said in an interview. In court documents, the school argued that the teacher’s married name was “the best way to identify an individual.”
“The desire of the plaintiff to use her commonly known name, which is different from her legal name, is not legally protected,” the school said.
Critics also say the law trails the private sector, which has been accommodating more women who prefer to use their birth surnames professionally. According to a 2013 survey of large firms by the Institute of Labor Administration, a think tank in Tokyo, nearly two-thirds of the respondents allowed women to use their given names at work, up from fewer than 20 percent in 1995.
In the United States, where women may legally keep their surnames after marriage, there is still a strong social convention among heterosexual couples for wives to take their husbands’ names. Even the highest estimates show only one in five American women keeps her surname when she marries.
“Naming is really one of the last socially approved kinds of sexist behavior,” wrote Laurie Scheuble, a sociologist at Pennsylvania State University, in an email.
In Japan, advocates for women, including lawmakers across the political spectrum, say they simply want women to make their own choices about their names.
“I don’t want to make a law where every couple has to use two different names,” said Kimie Iwata, president of the Japan Institute for Women’s Empowerment and Diversity Management. “But I want the society to make a law that is generous to everyone and accepts diversity.”
While campaigning recently for the leadership of the opposition Democratic Party, Renho Murata said she wanted to help change the marital naming law. (Ms. Murata goes simply by the name Renho, and her Twitter handle is @Renho_sha, using her birth surname.)
Seiko Noda, a member of the House of Representatives from the governing Liberal Democratic Party, said she had advocated the right to separate surnames for a quarter-century. “We lawmakers have to destroy this wrong practice,” Ms. Noda said, lamenting that many of her fellow Liberal Democrats did not care about overturning the law.
Ms. Noda’s husband, formerly Fuminobu Kimura, took her last name.
“Whenever he has bad luck, he always complains, ‘Oh, it’s because my surname changed,’” Ms. Noda said. She said he now supported an overhaul of the marital naming law. “He understands how much burden women had to go through,” she said.
Public opinion polls show a significant shift in views on married couples using a single surname. In 1976, close to two-thirds surveyed by the Ministry of Justice said the law was fine as is; that had dropped to just over one-third in 2012.
In the absence of legal reform, some couples choose not to legally register their marriages, even though women in such relationships have fewer legal protections.
Mizuho Fukushima, 60, a member of the upper house of Parliament from the Social Democratic Party who has repeatedly proposed a revision of the single-surname law, said she had remained in a common-law marriage for nearly four decades with Yuichi Kaido, a lawyer with whom she has a daughter.
“When I met him, I was Mizuho Fukushima, and my identity is Mizuho Fukushima,” she said.
A younger generation of working women say their original surnames carry professional clout that they do not want to lose when they marry.
“My name is my brand,” said Miyuki Inoue, 28, who works in sales at a human resources consulting firm. She married three years ago but continues to use her original surname at the office.
“I don’t want to waste the trust and good reputation that I’ve built in my career,” she said.