Tears welled up in Kaori Oguni's eyes as she recalled the heartache she felt at what was supposed to be one of the happiest moments of her life — that is, when she and her husband registered their marriage at a Tokyo ward office in 2006.

"As I was completing my paperwork, all I could think of was, 'OK, that's it then. This is officially goodbye to my family name,' " Oguni, a 41-year-old mother of one, said in a recent interview. "A family name is an important part of my identity. Losing it was painful."

Oguni is one of five people suing the government over what they call a "sexist" civil code that has traditionally resulted in Japanese women changing their surnames upon marriage. It requires Japanese couples to register their marriage under a single surname.