Japan has granted a long-term visa to an American man who married his same-sex Japanese partner in the United States, in what his representative called on Monday a "breakthrough" move in a country that does not recognize same-sex marriage.

The designated activities visa will allow Andrew High, who has been seeking long-term residence through the legal system, to remain in Japan for up to a year. The visa was granted by the Tokyo Regional Immigration Services Bureau on Friday.

"The decision was made on a case-by-case basis, but it should be established as part of the system," his representative said.

Same-sex couples are eligible for a designated activities visa if both of them are foreign nationals but not if one of them is Japanese. High had previously only been granted a temporary visa.

Last September, the Tokyo District Court said High should be granted the visa as the current system, which differentiates treatment depending on whether a partner is Japanese, was "not based on logic and violates the Constitution, which guarantees equality under the law."

However, it also upheld the denial of a long-term residence visa and dismissed claims for damages on grounds that Japan does not recognize same-sex marriages.

According to his representative, High and his partner began dating in 2004 and married in the United States in 2015.

High is expected to continue fighting in court for a more stable long-term resident visa, as the designated activities visa limits working conditions.