Former Osaka Prefectural Assembly member Kanako Otsuji said she will hold the ceremony at Ikeda Park in Naka-ku, Nagoya, at 4.30pm on June 3, according to Mainichi Daily News.
"I'd like as many people as possible to know that numerous homosexual people are forced to live with their partners secretly without being celebrated by anybody," she was quoted as saying.
Elected to the Osaka legislature in 2003, she helped change laws to make it easier for same-sex couples to rent public housing. She also fought unsuccessfully last year to keep a Japanese city from amending a rare law that had explicitly banned discrimination based on gender or sexual orientation.
In August 2005, she came out as a lesbian in her autobiography, "Coming Out: A Journey to Find My True Self."
"I thought I could give courage to the most people this way," she said.
The 32-year-old is set to run in the House of Councillors election in July through the proportional representation system on the ticket of the largest opposition Democratic Party of Japan. If elected, she would become the first openly gay politician of either sex.